As I am posting I am currently in the process of working my way throught the crematorium area of the medical pavilion in Bioshock. Not only is the game breathtakingly beautiful with the little bits of water work that just make me fall into trance. But the thing that truly brings a kaleidoscope of emotions is the well done, if not a bit jumbled when it gets a good bit of noise, ambient noise. Walking through the city of Rapture in a specific tube I hear what I can only assume is a Big Daddy and a little sister and I felt like they were right next to me. I turn around nothing. Continue looking and nothign again. Not only that I continually hear the rustic noise of a failing city the seaping of water. Simple closing my eyes gains me hidden depth in this rich environment. The voices of the crazed maddned indivual centizens dot my ears also. I love these noises the feel my brain with combinations of fear and other such things. Things like this are what made games like Slient Hill 2 for me. Just the fear of nothing being there when you heard something.
Ambient noise when down correctly can save a game from mediocrity or elevate it to new heights. Doom 3 is a bit inconstant with this but at it's best it does very well. You find yourself straining your ears for any warning signs, hearing far off fighting and fear for the tortured souls of hell.
The "hidden" Laboratories in STALKER did an excellent job of scaring the living daylights of me, although it was more the almost complete lack of ambient noise that did it (the Brain Scorcher in particular comes to mind). Amazing how complete silence with only the occasional clanging in some distant part of the complex or what sounded like the wheezing of a Bloodsucker for (dis)comfort can be such a mover.