using iTunes you can convert files to mp3. I'm not exactly sure how to do it in iTunes 7, but in iTunes 6 in Mac go to preferences >advanced>importing>import using mp3 encoder select the same bitrate as the m4a files>apply. then you can right click on the file(s) (select all you want to convert) in the iTunes library and select convert selection to mp3. When completed you can delete the m4a files if you so wish, although their quality will be that much better so you may wish not to.
I hope this helps you, even if you are on windows with iTunes 7 .. It should be roughly the same.
