HOW-TO: Convert .aac .m4a file to .mp3

You first extract the audio from the .mp4 or .m4a to .wav using faad. Then convert the .wav to .mp3

You need faad2 and lame installed.

Code:   

$ faad foobar.m4a       # to convert to wav
$ lame foobar.wav foobar.mp3          # to convert to mp3