(Sorry for bad English; it is my 4th language and I make a lot of mistakes in it.)
I had similar problem from the other side - as an older man, with a beard, some fat, and a small voice range, I have a lot of PCs and NPCs in my game (both male and female) that should be somehow believable. (I am unable to realistically make sounds like a young girl (not even a typical woman) and I would not dress as one, even if I am playing a female character - but it is not really needed to play one such way, that others accept her as what she should be like). Here are some tricks which I use.
1. I always describe the character first
And not only what PCs can see, but also how they are perceiving it. And it also takes a bit of voice modulation:
He sounds like a trader, even when he tries to hide that. His voice is
impressive, deep and full; you feel he could convince normal people to
believe him nearly anything. Make a Perception check.
- [if good] Wow, you can even bet that he worked in a bank or something
like that. He must have studied at a private college, probably [this
specific one], and trained his voice actively.
- [if bad] Not much, must be some banker or what. His voice was
probably trained, as nobody speaks that way normally - just people
from some special colleges, but you cannot tell which one.
(Yes, basically the same, but they discovered it and will see him
He greets you all formally, enjoying the convincing, deep sound of his
own voice, and says something like: [so far the GM was talking, but
now the GM is roleplaying the NPC's monologue; I take my voice down
like one tone or two, and speak a little more slowly] "Ladies and
gentlemen, it's a pleasure to meet you in person."
Then he looks at each of you, nods just for himself, and continues:
[and now I speak like the NPC - even more slowly, another tone down and
with all his personal traits] "I was told that you could use some
money and would not mind solving a little problem we've been having
lately. Some people need to be taken care of..."
Now, even when I speak only like three tones down (which is nothing difficult), my players perceive it as a much deeper and serious tone. And if I go back to normal, it is clear that it is the GM describing something, not the NPC speaking.
She is a teenage girl, with only a small bag, short leather skirt, and
a t-shirt with burning skull on it. She wears high heavy leather boots
with tons of spikes on them, and she has shaved her head to leave just a
high purple mohawk. While she tries to be seen as independent and mature,
her voice betrays her and you know that she must be even younger
than she pretends. She speaks really fast and enthusiastically, and
sometimes she needs to take a breath in the middle of her talking, but
she does not care and continues right away. [I start to speak a little
faster already, and I increase tempo on the way.]
Skipping greetings, she tries to get to the point as fast as she can, and
says something like [tone up and even faster]: "WoW! You really came and
it is just wonderful and I hoped you would come and I was worried, since
you don't know me and Mike told me you would come and Mike knows it as we
work for the maf... uhm, some people, and I should not talk about that and
there is some bastard and he must be punished and Mike told me to call you
and that you could do it and I don't have money, but I know something and
you'd really like to know it and I will tell you if you help me and that
[Finally out of breath, I gasp a few times and continue even one more
tone up] ...that bastard would be dead and mutilated and burned and
done forever and that thing is about something you asked about lately
underground and Mike told me about that and I looked into it and..."
[and you should have noticed long ago that she uses "and" instead of
any punctuation and especially at the end of sentences and starts new
ones and she quickly jumps from idea to idea and did I mention her favorite
word is "and" and she overuses it and does not stop talking at all and ...]
So while I was going only two small steps up or down in my tone, I made two characters that speak in totally different ways. Players usually hear them talking in a much wider range of tones, that I could even try to make, not to mention use effectively.
2. I set expectations for what players should hear
If I describe the voice in a certain way, they expect the NPC to speak the same way; they expect it to sound like the dialogue does in their head while reading a book. So I do not need to talk in a high/low pitch really, if I state the pitch and style before.
3. I usually introduce the character in 3 steps
First, I describe in my normal tone of voice what the characters see and hear. I might make players make some Perception checks (so they "reveal something" and take it as given as an important fact - if it is about the voice, they will be more inclined to actually hear it in the text).1
Then I summarise part of the speech of the given character (formally in my own words and voice, but I start to play the character already at lower force), and I alternate my speech tone and style a little towards the character's. (For instance, if I say, "She speaks about one or two octaves higher than usual, and talks about XXX," and I raise my tone just 1/12 octave higher for XXX, then it means I am talking about her style to distinguish it from my own style, but not that I am talking like her - I'm just summarising/hinting at her style of speech, after all.)
(And yes, it is cheating - I am really playing the NPC, while I deny it verbally. But it works, as it shifts the "mood" in the right direction. While I say, "No, it is not exactly what was said", so there is no expectation that I sound exactly like the character, players can get a "preview" of what was said about that character and how he talks.)
And then I make some break in my normal tone or describe something, so there is a visible/audible change, and then go to that character's direct speech with a slightly bigger change of tone/style. As players already expect her to talk two octaves higher, as I'd already hinted by describing her speech in a higher tone of voice, this is "official substitution" for what she really talks like. And If I use this tone (and style), their characters are supposed to hear "a tone of voice two octaves higher" even if I am speaking just two tones higher. (It's totally comfortable in my voice range to go two little steps up or down from my normal voice.)
(Also cheating - now I pretend that the previous talking about the voice was sufficient for anybody to hear that voice and that their characters were already hearing the right voice talking in the right style and tone as what I "summarised", so I now just establish an "official replacement" for what they already know, and both characters and players already totally understand what that replacement exactly stands for.)
1 A lot of my friends like to roll dice (as many as possible, and announce high scores as often as possible). Part of the rolls are "real" in a way that may or not reveal some useful hidden info; part of them are "just so" and they get some "hidden info" anyway, just served by different words, if it is good to have during the scene; and some are "just for fun" to reveal details that are just interesting, not useful (like finding out that some random spikes on her boots forms a particular star constellation (say, Pisces)) - this also many times turns to "conspiracy theories", which can make an interesting arc in the story themselves or give me some ideas that will interest my friends just now, and I could incorporate something like that (or subvert it).
4. I characterise more by the style of talking than by the tone
As you could see, the two characters want the same things, but they are totally different.
The businessman speaks slowly and in elaborated sentences and euphemisms; the teenager speaks as fast as she can in really short "sentences" and mixes everything together. They use different phrases ("Ladies and gentlemen" vs. "You"), and even different language ("Some people should be taken care of" vs. "that bastard would be dead and mutilated and burned and done forever"). Even if players just saw a sentence of dialogue written, they would be able to tell who said what and in which voice.
The businessman is always wearing a perfect suit, moves slowly, looks the same solid way, and has his two bodyguards with him. Every word of his is well-placed and premeditated. Everything he says sounds reliable, decent, and formal. These traits are exaggerated even more when he is actually wrong, lies, does not know, or a suggestion is plain stupid.
Lili just runs around, jumps, eats everything given to her, changes mood every day, and while she is intelligent and competent and has a lot of knowledge, she presents her options and recommendations in a totally careless style, and what she says sounds more childish or rebellious than it really is. These traits are exaggerated even more when she is right.
5. Repetition is the mother of wisdom
For a few following scenes/sessions, I would repeat the voice characterisation before actually using it, just in a shorter way:
You hear Lili's high and young voice at her typical unbelievable rate
from around the corner as she says: [at a higher tone and speed]
"...And you are ugly and I will not help you and I will kick your ass
and stomp on your corpse if you ask again and it serves you right to
get that and..." (At which time usually somebody's character screams,
"Not her again!" - so the player remembered who is speaking, and his
character recognized that already and remembered all her manners and
the problems with her.)
6. Trademarks are important
It is good to have some trademarks (or quirks or so) for each important NPC, so that just a few words is enough to recognize them and are somehow characteristic for them.
Lili talks fast and abuses "and" to no end.
Cat is a young woman who is a cat shaman. She talks like a cat as much as possible. She does not say "Hi" or "Hello", but sings "Ci~a~o~o" in a way that sounds like "Miaow" and instead of "OK" she says "Fi~i~ine" (again like Miaaw - well, it is a bad translation, as we play in Czech language, where it sounds even much more like a cat speaking; I am not exactly sure how cats speak in English) - especially with her imitating it by tone too and using many words that can be pronounced as cats do.
A troll warrior uses preferably one-syllable words (or even sentences) "I. Hit. Him. He. Dead." (Rarely using words with more than two syllables, even.)
Genja - an orc from Russia - constantly puts the phonetic stress in the wrong part of a word. He talks slowly and like it is hard for him.
7. Gender stereotypes
Until totally formal, girls and boys talk and think a lot differently. It is cultural and all, but it can be used for characterisation too. (Well, we are playing archetypes after all.) On the other hand, if it is used the wrong way (accidentally using a female stereotype on a male or otherwise), it stands out a lot and either breaks the illusion or suggests strange things about such a character.
It is much easier to disrupt the picture of a character of the other gender by using your stereotype, instead of his, than to disrupt it by overdoing the stereotype or talking in a bad tone.
"What about the bar?" - "Hell yeah!" / "Hell no!"
"What about the bar?" - "It would be really pleasant, if you do not mind."
/ "I do not feel like that just now, but it may be just me."
If playing a tough male NPC, throw in some convenient phrase(s) that he would use a lot.
If each of his lines starts with "Damned, ..." (at a place where he is suppose to take a breath and imagine what he wants to say), it is usually believable and it does not mean anything for the rest of his speech. Or any other curse/profanity/word parasite ("By the horned devil,... ", "Raxos' cursed eyes, ...", "Rusted crowbar, ..." or anything like that). Like "Ehm, ..." or "Sorry, ..." would work for a shy character as the normal start of a sentence. Or all those, "Eeee, well, I think, I would say..."
The talk can then go in any direction; it is just a way to collect one's thoughts. It can also be good if he is stuck in the middle of a sentence, or surprised, or so.
Also, a friend of mine was just here and told that when playing a male warrior, the NPC should not talk about his feelings, or talk just to keep talking, or to make others feel good (does not count on showing sympathy by cursing - "Yes, that sucks" and such) - it would sound wrong nearly every time. (It did not even come to mind that I would do that, but when I play a female NPC, I take care to make her talk a lot about how she feels; while playing a male NPC, I do not even think about doing something like that - the male NPC just talks about what they did, or what they will do.)
Also, female NPCs usually take much more care about how they look, what they wear (for beauty), and how they are viewed by others. Male NPCs tend to talk about technical stats, what they would take on (for best bonuses), and easy make mistakes like "I cut its head with axe, take it both hands up and go show around the city - But the head is big and the blood runs over your hair, face and all your clothes - Oh well, it does not matter - Citizens are running from you, screaming - Why? I saved them!" (This really happened.)
They offer you new high heavy black boots, yellow overalls, and a green hat, but you do not want to take it at all; what do you say?
"No, thanks - black, yellow and green does not go together!"
"You are insane - yellow doesn't provide any camo in the jungle!"
Even if my female NPC wants to look hard (or rebellious) and curses, they use much softer words than my male NPCs. Not only because it is "feminine" (I know girls who normally use language that I would not use), but because I am creating an illusion and so I am bound to "stick with the rules" to not break the illusion of a female NPC, even if a female player could do it anytime. On the other hand, I can play male NPC as I want, while a female GM should "stick to the rules" to not break the illusion of a male NPC.
This does not prevent them from being really cruel or brutal (if they are); I have just use other wording when they speak.
I was hungry and there was a boar, so I snuck there and cut its head off
with an axe. Then I skinned it and roasted it on the fire. Then we all
I had seen a wild piggie and it was so dirty and there was a nice clean
river and so I used some spell and levitated it there and washed it im
the water and it made a lot of funny bubbles and tried to swim or whatever
and the water washed it really well and it then stopped making the bubbles
and I took it out and it did not move and I did not want to put it
in the dirt again and so I took it inside and it still did not move and
it would rot anyway in such a hot jungle and so we somehow preserved
it and so I made a nice fire and we had a great picnic and I made a
hole in the ground and we properly disposed of all the remains there and
so the place looked the same and there was no harm to nature at all...