m4a format is also known by other extensions depending on the player you use. eg: Winamp uses mp4. The problem with that is such an extension can also be mpeg 4 video. iTunes Music Store use m4p (p=protected.) m4a = mpeg 4 audio. mp4 audio & m4a extensions are interchangeable. Nero even confuses it more because of Nero's use of mp4 as DVD subtitles (Vobsub). Nicely explained here http://en.wikipedia.org/wiki/M4a