Guys doesn't matter what sound card you have, this game uses Xaudio2 which channels all the sound through your CPU, you sound card never gets involved, it is one of the reasons why it runs so badly for some people.
I have noticed recently that the enemies don't make as much sound as they used to, I do hear them sometimes but not as often as I used to, perhaps TWI reduced the sound calls in the patch to improve performance, they are still there sometimes not always though, I can even hear people creeping around if i am still and there is not much background noise.
Try Turing your channels to 64 (I wouldn't go to 128, to much of a hit on your CPU)
It is a shame they didn't use OpenAL to do the sound, would of been much better and would of used the hardware for those that have it. It would of also been smoother and the sound would of been better all round, especially the positional side of the sound. I have an Xfi and rather miffed about this