I can't even begin to tell you how much I love the sound in this game, it's so interactive, accurate, and refined compared to a lot of games out there. For the most part the game sounds "mesh" together exceptionally well to create a good ambient and environmental feel. I'd really like to give props to the people who did the surround sound as well. The number of times I've been able to detect and kill cloaked Recon's purely because I can hear their footsteps is awesome.
I think this is a really undervalued part of the game, and I just wanted to give my congratulations to the person or persons in charge of getting it bang on! I don't normally notice much of a difference switching between surround and stereo in games, but it's a must in this game!