My Ingame sound suddenly stopped working?(vista)

I'm about 1/5 into the game using vista, and the sound suddenly went out. The weird thing is, if I load a game before it went out, the sound works fine. You can still hear blaster fire and "get sum" talk during fights, but all cut scenes and dialogue sound is not there. I've tried going back and playing to before it, but every time I go through a certain area the sound goes out.

I recently downloaded the vista compatibility, but it worked fine. Is there any reason its not working now?

thanks for reading, please help.