http://gimpel.gi.funpic.de/Howtos/convert_aac/index.html

Howto: convert aac/m4a to wav/mp3/ogg on Linux 
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. For the conversion you need the following tools installed:

    * mplayer (with faad2 support compiled in!)
    * faad2
    * for mp3: lame
    * for ogg: oggenc (vorbis-tools)

The conversion is done in two steps:

1. aac/m4a to wav

In case you only want to convert a single file:

mplayer -ao pcm inputfile.m4a -ao pcm:file="outfile.wav"

If you want to convert all files in a directory, you can use this little script:

Linux/Musik/Formate/aacm4a (last modified 2008-11-04 06:59:55)