I was going to also suggest Super, but I hadn't heard of what M4a file was. Turns out that it is just a Audio file that uses MPEG-4 audio compression, so as saiko suggested, SUPER AV Converter is good
or just burn it to a disc with itunes and rip it back to mp3 if you have an mp3 encoder, but that was back in the days and the hard way haha...go with the SuperAV converter...