Did you disable the onboard sound as I mentioned earlier? It sounds like a sound conflict. Also, are you running a custom sound pack in-game? If so remove it and see if that helps.
BTW, if you need more than 2 Gigs of RAM, stick in 4 matched 1 Gig sticks. Windows will only recognize 3-3.5 Gigs of it, but it will remain in dual channel mode (double the speed of single channel).
I doubt the following will make the difference but I also have to ask if you've D/L'd the latest drivers for your video card and installed them and if you've gone to the NVidea control panel to set up the card. Most of the set-up items give you a short description of what they do. I mostly stuck with the default settings (512 Mhz 8800 GTS) although I maxed anti-aliasing and a couple other settings I allowed the application to optimize.