I have some .m4a files that were captured from a radio broadcast stream. They are playable using mpd (an open source Music Playing Daemon for *nix), but iTunes won't open them (nor will several other music players that should be able to play .m4a files). When I looked at the file in hex format, and compared it to an .m4a file from iTunes, it appeared that the file from the broadcast stream did not have any header info.
So I figured I'd try prepending some header info to the file, to see if that would make it playable by these other players.
I've tried to read the technical doc about the .m4a file format to understand how the header is structured, but it's far too complex for me to follow (most of the doc is about the coding, which isn't pertinent to this). The header seems fairly simple, conceptually - I can see 4-character tags, variable length data, and there are presumably some length codes to allow these to be split apart. It doesn't seem to contain any significant data about the actual audio data (like its total length), making me hopeful that I might be able to get by by simply copying the header from an .m4a file and prepending it to these headerless files.
I'm unable, though, to manually parse (that is, reverse-engineer) the header in order to even experiment with this. Can someone describe how to identify a complete header at the front of a .m4a file?