It sounds like it must be an issue with your computer then. Could be either system softw or hardware such as your sound card. I know there's a control panel for setting your system sound, it could be well worth checking it out to make sure there's nothing that's been changed. Perhaps an effect that's been added. Why don't you try putting an audio cd in your computer & try playing that.

Have you done a system maintenance lately? Defragment your hard disk drive, check files with file utilities, etc. Checked your comp for spyware, viruses etc.