darkstriker: Having the same problem, myself. I've tried: Rebooting, changing from 7.1 to 5.1, old saves, changing voice over language, and using headphones.
It seems to have happened after I loaded a save game in the middle of a dialogue, and after the game randomly exited.
UPDATE: I managed to "fix" it, but I'm not sure how. I moved my saves out of the directory and started a new game, just to see if that would work. Strangely, it did. I moved the saves back into the directory and loaded one, but the sound didn't work again. So, on a whim, I went and started a new game again. After the sounds worked there, I just loaded one of my saves (without quitting) and it worked fine. Yes, convoluted steps...but they seemed to get me ambient sound and voices back.
And again a big thank you to a participant in this forum. Had the same problem and the "fix" worked like a charm here too.
One of the weirder bugs I ever encountered...