So I just beat the DLC "Point Lookup" and talked to several merchants (to sell some times). Now I've noticed that all dialog sounds are suddenly gone. I was able to complete the full game including 3 out of 5 expansions without this bug ever occuring once. I have already restarted the game and tried loading different savegames - but the problem persists.

Any help is greatly appreciated. Just in case someone raises this question: I'm playing Fallout 3 without any custom content installed.

Well, I seem to have fixed that "bug". My speaker options were set to Stereo, thus causing my center one to become muted.