There are various sound problems and some workarounds have been suggested ....
Have you ever had someone talk on squad channel and you eventually notice that things got much quieter and you look up and notice that one of your squaddies name is stuck in the transmit display area ?
Simply drop back to the desktop (I use that funky "windows" key on my keyboard) and then click back on the game and that condition should clear itself.
I use to have my mic drop volume on a consistent basis ... again I would have to drop back to desktop, and un-click Mic Boost and re-click Mic Boost to solve that problem. The problem would then manifest itself again within 15 minutes or so ... what a pain in the ass.
I read another thread that suggests that you drop your sound acceleration by one notch. To do that you execute DXDIAG from the RUN window. Once the DXDIAG Panel has been displayed, click on the "sound" tab. If your sound acceleration is cranked all the way up, drop it back 1 notch. I have not had any problems with losing voice volume since doing this.
I am not an audiophile, so turning down sound acceleration is not a problem for me ... and when I do play music or DVDs on my system, its sound fine to me.