It's possible that you've synced music that's not compatible with your iPod or the music you've transferred is corrupted. To check for the official file formats, access the Apple page created for this purpose. Before attempting to play or sync the files, connect your iPod to the PC and start iTunes. Click the iPod's name and then check for updates. Firmware version 1.3 should be available for downloading. Start the update process and then sync the music files.
