Do you shut down all non-essential programs before starting Divinity? If you exit the game and start it again, does that fix the sound problem? How about re-booting?
Try turning the sound hardware acceleration down (Start | Run, type in dxdiag and hit Enter, then switch to the Sound tab of the DirectX Diagnostic Tool). I don't know if that will help, but other than updating your sound drivers (the Device Manager in the control panel should list what you have) that's all I can think of to try.
