Weird. I know other people's m4a's were being played in QuickTime instead of iTunes, but they all work for me. WAVs also go to QuickTime, but wma's won't work (as I'm not on a PC). I don't know why they won't work then, so I'll try to fix it.
EDIT: I don't know how else to convert it, but it says it's encoded with iTunes and QuickTime, if that makes a difference.