Hello guys and gals.
I just encountered the sound problem recently on Battlefield 3 pc which I suppose most of other people might have encountered too. It is not the one of sound cutting out or missing during the game, which in my opinion is due to lag on some busy server. If you by any chance do not hear in-game sound while playing battlefield 3; however, able to communicate with a team using team speak then the solution is simple.
While playing a game go into window mode.
Click the sound icon on the desktop (i.e bottom right corner).
Click the mixer (Windows 7) and there you will see the sound level bars for speakers, microphone, TS and Battlefield 3.
Crank the volume all the way up for BF3.
N.B. You are most likely to do the same most of the time which could be due to key conflict with Team-speak client.