Is it like "warbling", full of "glitches"? A bad mp3 file? Are you using any mods. What springs to mind for me is that you have a sample mis-match, between the software music and the hardware audio card and / or usb audio device. The game uses mostly 44.1khz audio, and your computer might be set to something different, and is getting converted "on-the-fly", but not at a quality level.
