MP3 Scan + Repair.
This little application is a must have if you download MP3 tracks from the web (outside of the usual sources such as iTunes or Amazon MP3) or are finding that some music files are unplayable on your system. This FREE app basically does what the name of it implies; it scans any MP3 files you drag and drop into the window (or icon) and if desired, you can repair/rebuild the file(s) selected.
What I've found out is sometimes MP3 files get damaged and is no longer recognized as a valid MP3 or sound files. This may have happened during a copy or download. Basically the resource forks (or something) within the file are damaged, thus altering the file in a minor way and making it unrecognizable as a music file.
I use MP3 Scan + Repair on nearly every MP3 file I obtain from an 'outside-the-norm-source' to ensure the files are clean and coded properly. Overall, a very useful tool that I have in my arsenal of Mac file utilities.