Watbol again... I hate to be a Dic', but if all you want to do is convert m4a to mp3, X-Recode will do it.
http://xrecode.com It's free, and supports opening: mp3, wma, ogg, flac, ape, ac3, wv, mpc, cue, tta, cda, wav, dts, mpa, mpb, mp4, ra, rm, aac, avi, mpg, vob, mkv, swf, flv, and mov.
Some of these formats I have never heard of, but it can do them. Many settings for each format. Although it can read all of them, I
think it can only write:
mp3, wma, ogg, ape, flac, WavPack (.wv), aac, alac, wav, m4a and multi - (If you need a song in more than one format simultaneously.)
As these formats are in the opening screen... and selectable.