Go to Tools>Options>Audio>Settings and see if the box is checked that says "Play files from memory instead of disk". If so uncheck and try.
Make sure to reboot first. It's possible that your audio driver is just borked up.
Unchecking Play from memory, if enabled, might be a good troubleshooting step, as that isn't really a recommended option. But reboot first. Other troubleshooting steps:
http://wiki.jriver.com/index.php/Audio_Troubleshooting_GuideIf rebooting doesn't help (or especially if it does, but the problem comes back later), then my first suspect would be an audio driver.
It could also be disk access issues, though. If the files are on an external disk or network volume, or whatever, try copying some over to C as suggested in the article, and see if the problem goes away. If so, it is something wrong with disk access, and the
Troubleshooting Disks article is where you want to go.