I helped a friend move his iTunes library from an old PowerPC PowerBook to an Intel MacBook Pro. We followed the method as per http://support.apple.com/kb/ht4527 and used an external drive to transfer the library across. It all seemed to be fine, but any m4a file will not play. No error, no nothing; they just won't play. They won't open in QuickTime or VLC either. And it's not a case of the files not existing or being in a different place. They are present and correct and iTunes is referring to them correctly. It's not an authorisation issue, either; the computer is authorised, plus many of the tracks are not purchases anyway, but rather CDs that were imported into iTunes on the old computer. Any ideas? I'm at a total loss!