I just wanted to add something...
I have been experiencing the same problem, even with the same game xD
other games did fine, only Cod 4 had the problem, and same for me (that's why im really glad that i found this topic) it was the soundcard SoundMax HD, which is the onboard one, the one that i also had SB X-FI Audio [9C00] does help the problem too. the only problem stays that i want to listen music over my 5.1 Speakerset which can only fit on the mainboard soundcard. And i am unable to run both soundcards at 1 time, since i have to switch the soundcard options in my Audio settings...
but anyway the problem is fixed for me too :laugh: so thnx a lot!