I have an MP4 of a live performance and I want to put the audio from it on my Sony Walkman. I opened the MP4 in Quicktime Pro and deleted the video portion, leaving only the AAC audio, and saved it as a MOV. I then changed the extension from MOV to M4A in the Finder. I imported the file into iTunes, and it plays just as you'd expect, but when I dragged-and-dropped it onto my Walkman and attempted to play it I got a "cannot play, file is damaged" message. Despite the M4A extension, the document icon has remained the Quicktime one, and when I click Get Info it lists the kind as 'MPEG-4 Audio'. For an iTunes Store file or CD rip the M4A the icon is the iTunes one and the Get Info kind is 'MPEG-4 Audio File'. So my questions are, what's the difference and how can I take the audio portion of an H.264 video and turn it into a bona fide M4A (without recompressing it)?