I have a Mac there's a little program you can download called Switch which enables you to convert various sound file formats to each other. It takes a couple of minutes to convert an m4a to mp3.
Switch is trialware, ie after a certain number of uses it locks and you have to pay for it (can't remember how much it was). However, I'd be surprised if there wasn't a similar free product, for Mac or PC.
agreed..there are many free conversion programs for The PC and MAC
C-net should be of help.
http://index.about.com/index?q=Free+Convert+Mp4+to+MP3&qsrc=6&o=29572&l=sem&qo=relatedSearchNarrow