Are you running the game with a 5.1 speaker system?
Could you provide some information on your computer/> Dxdiag will probably do it. To produce it, press Start - Run, then type in "dxdiag". "Save information" to the text file when finished.
It is the problem that arises from time to time, and each time I forget how to solve it.
Maybe just because I have never encountered it on any of the computers I had or happened to work on. The solution is something along the lines of reinstalling your Realtek drivers (if it's the case) or OpenAL that seems not to work good sometimes - or, perhaps, it is that the version on game DVD is not the correct one.