After reading the rules related to hiding several times, it seems like when hiding you have to keep quiet, and thus detecting hiding creatures by hearing alone doesn't work.
I see a lot of places in the rules where this is assumed, but I want to confirm that my reading is correct. A lot of these passages don't make any sense if I try assuming that hiding creatures can be detected by sound.
The rules for hiding state:
You can't hide from a creature that can see you clearly, and you give away your position if you make noise, such as shouting a warning or knocking over a vase. An invisible creature can always try to hide. Signs of its passage might still be noticed, and it does have to stay quiet.
It seems to me If you are hiding, you are quiet unless you are making noise as part of the requirement of hiding, along with "not being clearly seen".
The rules go on to say:
In combat, most creatures stay alert for signs of danger all around, so if you come out of hiding and approach a creature, it usually sees you. However, under certain circumstances, the DM might allow you to stay hidden as you approach a creature that is distracted, allowing you to gain advantage on an attack roll before you are seen.
The rules specifically say "seen", so it seems to me the rules continue to assume that a hiding creature is not making any noise.
The rules go on:
What Can You See? One of the main factors in determining whether you can find a hidden creature or object is how well you can see in an area, which might be lightly or heavily obscured, as explained in chapter 8.
This seems to me to be very clear text: how well you can see the creature affects how easy it is for you to find it.
Lightly or heavily obscured areas
The rules for lightly or heavily obscured areas says:
A given area might be lightly or heavily obscured. In a lightly obscured area, such as dim light, patchy fog, or moderate foliage, creatures have disadvantage on Wisdom (Perception) checks that rely on sight.
A heavily obscured area--such as darkness, opaque fog, or dense foliage--blocks vision entirely. A creature effectively suffers from the blinded condition when trying to see something in that area.
Both of these conditions are purely visual, so it seems to me to indicate that searching for hidden creatures is usually about vision -- the rules say this is "one of the main factors", so it seems unlikely to me they meant "in the rare event when a creature relies purely on sight".
I also looked at the description for Perception:
Your Wisdom (Perception) check lets you spot, hear, or otherwise detect the presence of something. It measures your general awareness of your surroundings and the keenness of your senses.
Again there I see the normal division of senses: see, hear, and other (taste, smell, tremorsense or exotic senses)
The rules go on to give examples:
For example, you might try to hear a conversation through a closed door, eavesdrop under an open window, or hear monsters moving stealthily in the forest. Or you might try to spot things that are obscured or easy to miss, whether they are orcs lying in ambush on a road, thugs hiding in the shadows of an alley, or candlelight under a closed secret door.
There are examples of both "hearing" stealthy monsters, and "spotting" hiding thugs.
Hiding is a specific mechanic with its own unique mechanics, a subset of stealth, although it requires a stealth check. This again seems to me to imply that "hearing" isn't useful for detecting a hiding creature. As further reinforcement there is the mention of "in shadows", which is visual.
Unseen attackers and targets
At the start I noted the benefit of hiding is that you are an unseen attacker:
Combatants often try to escape their foes' notice by hiding, casting the invisibility spell, or lurking in darkness.
When you attack a target that you can't see, you have disadvantage on the attack roll. This is true whether you're guessing the target's location or you're targeting a creature you can hear but not see. If the target isn't in the location you targeted, you automatically miss, but the DM typically just says that the attack missed, not whether you guessed the target's location correctly.
When a creature can't see you, you have advantage on attack rolls against it. If you are hidden--both unseen and unheard--when you make an attack, you give away your location when the attack hits or misses.
There are several things I noticed here: - It gives the example of "lurking in darkness" again, which is purely visual. - If you can't be seen, there is disadvantage on the roll. - If you can't be seen or heard, you have to guess the location, and then you have disadvantage - Hidden is called out as "unseen and unheard", once again matching up with everything else I read.
When wearing heavy armor you have disadvantage on stealth checks. I understand this as a combination of armor being noisy, bulky, and awkward.
Heavy armor makes it more difficult for you to be stealthy, but it doesn't give advantage on perception checks to detect the wearer. So a roll of 20 with heavy armor and a roll of 20 without are the same, it's just harder to be stealthy with the armor.
After a careful reading of Hide, Hiding, Stealth, Perception, and the Light and Vision rules this has lead me to believe that it is expected for readers to understand that hiding creatures are quiet.
Although I have investigated every rule discussed in all sections relating to hiding, but there may be something else somewhere. Is there any other information (features, monsters, official adventures) which indicate that you can detect a hiding creature with hearing?
This could also be just a coincidence. Perhaps WotC chose similar examples and was a little sloppy on some wording, leading to the wrong conclusion being reached consistently. Is there any indication from designers that hiding creatures are intended to be detected by hearing?