I don’t believe you can convert them as they are DRM protected.  There was a program awhile back called Playfair that broke Apple’s AAC protection, but it was taken offline and is kind of hard to find now.

I know there are free programs out there that will convert from MP3 to MP4 or 3GP, I just don’t know if the will convert from MP4 to MP3.  Here is a Google search link on them, I don’t know exactly what you want so knock yourself out!  http://www.google.com/search?hl=en&q=mp4+converter

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