Try going into the Control Panel, and then to Sound And Audio Devices. Try then going under Speaker Settings to Advanced, and checking that the right surround has been set for your speakers. This can sometimes cause an error in games causing no sound (Doom 3 did it with me) if its set up wrong here
5.1 Surround Speakers I think should be checked.
If this is working fine, check in the game options for a surround option, checking that it has been set correctly.
I hope this helps