Anyone can repeat phrases
if the 2nd character now says several Orc phrases over the course of a minute, Hansel can repeat each phrase at will, mimicking the voice of that Orc.
If I explain to you that "buenos días" means "good morning", "gracias" means "thanks", and "¿cómo estás?" means "how are you?" then I'm sure you can repeat them back to me.
If you have ever been to a foreign country or even interacted with someone who speaks a different language then I'm sure you have had the experience of asking or being asked "how do I say X". This is something that anyone can do.
Actor lets you imitate voices
If you are good at imitating voices then I'm sure you can try copy my exact voice. Give it a try, go to google translate and type something in then try to copy the voice. Even though it's another language I'm sure you can get pretty close to realize that someone skilled could do it. That's the extent of what the Actor feat can do.
Creating new phrases requires knowledge of individual words, if not grammar
Joe believes that Hansel can also rearrange the words to create new phrases, on the fly in conversation with Orcs
This is where the plan falls apart. Even if you have memorized how to listen for and respond to 100 phrases, you are going to be out of luck when hear something not on the list.
If you know what each word means then you may be able to swap "I like cats" for "I like dogs" easily. But there could be all kinds of unknown grammar rules that you have no idea about:
Tanner Swett said:
In Polish, "This cheese is good" is Ten ser jest dobry, "I like this cheese" is Lubię ten ser, and "This dog is good" is Ten pies jest dobry.
So how do you say "I like this dog"? It's Lubię ten pies, right? No, it's actually Lubię tego psa.
Now I'll give you another sentence. "I don't like this dog" is Nie lubię tego psa. What is "I don't like this cheese"? It's Nie lubię ten ser, right? No, it's actually Nie lubię tego sera.
You may be able to decompose something you heard and try to guess the meaning, you may be able to swap out a few words to slightly change sentences, but if there is more than a minor difference, then you will need to know about the grammar structure of the language, and that isn't an easy thing to know.
Speaking slowly with broken language and a soundboard isn't convincing
- and that this can also fool any Orc into thinking it is an Orc from their own War Band, so long as there is a door there to muffle the sound.
Imagine you are the Orc, hanging out at your War Band's hideout. You knock on a door and you hear a voice that sounds Orcish (though not a voice you recognize) repeating back phrases like a sound board: "Hello. I am your friend Grog. I am a fellow Orc from your War Band. I enjoy cheese".
If you ask a complex question they pause for a while then reply in broken English: "What kinds of cheese do you like?" "... Yes. Yes I cheese. Enjoy much for cheese the. Cheddar."
So, are you convinced? Must be Grog, maybe he ate some bad cheese that's why he sounds funny?