A solution that has worked for me in both vanilla and FA is to switch the audio settings from 5.1ch to stereo.
I did this by bringing up my SoundMAX control panel and used the wizard to switch from 5.1ch to stereo.
Just wanted to throw in my two cents and cause this post to be bumped. After TONS of trial and error with Supreme Commander on three different sound cards this weekend, I have also found that I MUST reduce my audio settings to Stereo to get decent performance from Supreme Commander (patched to 3280). I am only running an AMD Athlon 2800+ (single core) with an ATI 2600 XT vid card. However, I can get 40+ average FPS on the perftest with video details set to medium by changing the sound settings to stereo. Without the change, my average FPS will drop into the 20's AND performance will be unbearable jerky when there's a lot of firefighting (and sound effects) going on.
Patching from 3189 to 3280 takes average FPS in perftest map out of the 30's into the 40's. Also, game always runs better in XP than in Vista.
Hope this helps somebody else from having too much grief!