Resonance and timber combined with delay and echo tell the listener how large or small a room is by the echo, and what kind of material the room is made of by the timber and pitch of the resonant sound bounced back to the listener's ears. When you stand in a metal chamber, the resonant sound is tinny and echoed, whereas in a normal household room the sound returned to your ears is easier on the ears and has very little echo because all the furniture absorbs the sound. This is why sound is so important to gaming. We have said it before and we'll say it again. Very good, readers.
Sound paints the scene for the mind in a far more subconscious way than visual input ever can.
Sound tells the player whether they're outside or in an underground cave, beside water or on top of a mountain. True 3D sound tells the player if the water is on their left or right, if the door that is opening is above them or below them, or if a freaky alien is coming up behind or in front of them. The trouble is most computers have only two speakers, both in front of the player. This makes it hard to get a sound to realistically come from behind the player's head. True surround needs five speakers.