You'll need lame, mplayer and a python library called mutagen.

It should run wherever the requirements are met. I run it into a virtual machine running Slackware on my MacBook.

Run the sh script passing the name of a directory where it can find some m4a file in and it will decode to wav, encode to mp3 and copy some of the m4a metadata to the mp3 id3tag. Finally it will clean the dir from the intermediate wav files.

You can change the options passed to LAME from the bash script. Read the comments.

I don't support this set of scripts but I hope you'll find them useful. If you want to improve or fix stuff, http://github.com/alieb/m4atomp3 fork, create an AUTHORS file, stick your name in, do your stuff, notify me with a pull request and that's it.