The M4P format is proprietary to iPhones/iPods. I needed to modify some songs that I got from the iTunes store. First I used Sharepod to remove the M4P files from my iPhone. Then I used SoundForge to record the song from Winamp playback. Winamp does not play M4P files natively, so you’ll have to install the M4P plugin. Also, you may have to run the Windows Master Volume tool to properly unmute the audio source.
If you want to burn M4P files from iTunes, create a local playlist, and drag-and-drop the M4P files you got from Sharepod into the local iTunes playlist.