Then, of course, there's music. Your music says as much about the feel of the game as any other element of it does. Again, if you've been painstakingly realistic in the creation of your gaming environment, your music had better complement the effect that you've created. Similarly, getting the right music can keep the player totally immersed and involved, enough so that the player forgets to eat. Sometimes the best use of sound is NO use of sound.
Silence is at the heart of suspense.
If you've decided to incorporate human voice into your game, you've opened up another can of worms. Once you've answered that string of questions, you have to find, audition and choose voice talent. This is all before a single line is recorded. Oh yeah, find a recording studio. None of this is cheap.