You need to make sure your audio is configured to stereo 16bit 44100MHz.
first set it to stereo, you do this by opening the old windows audio settings menu (go to configure) :
Now select Stereo :
Then you can set it to 44100 Hz in the "Advanced" tab :
there. you're now in stereo audio but there's a good chance this won't be enough if the first time you installed and ran FA, your computer was configured to use surround sound audio.
to persist these changes into FA, delete your Game.prefs file at (warning you'll loose your FA user config for example what mods are enabled, what settings you set, build templates) :
%LOCALAPPDATA%\Gas Powered Games\Supreme Commander Forged Alliance\Game.prefs
and re-create it by running Forged Alliance from steam and re-creating your username.
Now FA understands that it is in a stereo environment and audio will work.
You can remake your templates, and add your settings and ui mods back.
The bottom line is Forged Alliance and 5.1 or 7.1 surround sound are incompatible as FA was built before these techs were on the market so you have to choose.
however, in my experience. so long as FA is shut down when you switch your audio back to 5.1 or 7.1 and you switch your audio back to stereo before running FA again, everything works out.