its down to the audio settings. your trying to run it with 7.1 surround when it needs to be on 5.1 or 2.1 depending on your headset/speaker setup... even if you have 7.1 surround speakers you will get sound distortion as it trys to send sound to the wrong speakers... so switch the default speaker setup to a lower setting... also switch from 44khz down to 42khz (i think, well its the next 1 down regardless) if you can.
Yeah, I've tried all that - I've got onboard 5.1 audio, but I use a 2.1 setup. Tried changing all settings from Very High on down to Low, and it still pops and crackles with ballistics and explosions. Also tried changing the speaker settings in-game using commands for headphone setup, surround, stereo, mono. There seems to be less when running the game in DX9, but they're still there. I gave up last fall.