http://gimpel.gi.funpic.de/wiki/index.php?title=Howto:convert_aac/mp4_to_wav/mp3/ogg_on_Linux
Howto:convert aac/mp4 to wav/mp3/ogg on Linux
From Linux Multimedia Wiki
m4a is the file extension for the proprietary audio format mostly used by that iTunes thingy. A conversion to more player friendly (usb mp3player for example) formats like mp3 or ogg only works, if the aac’s aren’t DRM protected.