there is a program called "ffmpegX" for mac. it will change more or less any audio/video file to any audio/video file. you can google it and download it for free, but at every launch it asks you to pay $15 for it. dont bother and just click "Try for free". the only problem is that it does not work alone, you have to download other, smaller programs to get it to work. it tells you the names (there are 3). just google them, download them, hide them somewhere away on your computer, and then just tell ffmpegX where to find them.
good luck to you