Convert m4a (which is mp4) to wav  (and then onto mp3):

• mplayer -vc null -ao pcm [input-filename.m4a] -ao pcm:file="[output-filename.wav]"
• or:   faad [input-filename.m4a]

Convert from WAV to MP3:

• lame -h -b 128 ["input-filename.wav"] "[output-filename.mp3]"