Are you getting the dreaded "The file you attempted to play is not a valid MP3+G ZIP file."?  More often than not, this happens when a file download was interrupted.  Many of these cases can be solved by simply re-downloading the file.  To re-download the file, follow these steps:

1. Delete the invalid file from your library.
2. Visit the "View Purchased Tracks" section in the application and click the track you wish to download.
3. Wait for the download to complete
4. Visit "My Library" and attempt to play the file