Could be one of two things:
Your OC is unstable, (or PCI freq too high if using onboard sound);
Or more likely you have downloaded files that have been tampered with. I have noticed this too. All my old files play fine. When I do a search for a song, and I choose the file with the most users, even the file looks ok, it plays like crap once you get a few seconds into it.
RideGuy