Battlefield: Bad Company 2

Early on in Battlefield: Bad Company 2, during a mission that the ESRB has somewhat spoiled, you hear an absolutely frightening and ominous sound. Its inspiration can easily be found in things like a foghorn or the tripod from Steven Spielberg's version of War of the Worlds, but even so, it's still a surprising and unsettling noise. The way it completely assaults your speakers and rattles your subwoofer serves as a perfect example of some of Bad Company 2's phenomenal sound design -- one of the many things that developer DICE has gotten right this time around.
Sure, there are a lot of improvements (and even a few missteps) within Bad Company 2, but the sound design is particularly noteworthy. It's not just that the guns sound realistic (as far as I can tell), but that DICE's sound gurus have tweaked, amplified, and reverbed them enough to sound terrifying. Sniper rounds carry an ominous thunderclap in passing. Assault rifle bullets alternate between cracking the air and forcibly puncturing whatever surface -- flesh, wood, stone, or metal -- they impact. The way a light machinegun erupts during gunfire indicates that it's designed expressly for the purpose of murdering your enemy. Other sounds, such as the crunch of footsteps in the snow, the creaks of collapsing buildings, or the chattering of jungle insects, contribute to what is one of the best soundscapes in a modern FPS (especially if you set your audio to the "war tapes" soundmix).