I also got some problems with mp3 files.
You can use ResEdit in order to change the format :
Open ResEdit (if you don't have it, go to apple's website -> Download)
In the File menu, select "Get File/Folder Info..."
Then select your file
Type = Mp3
Creator = TVOD
Then save and quit.
Your mp3 file will be recognized as a QuickTime Player mp3 file.

Good luck :-)