VLC was supposed to be able to open M4P files by reading the authorisation key from an iPod, and could then output the audio directly to a file, but it doesn't work any more, and neither does FairKeys.
This indirect method doesn't preserve the tags, but it's useful for converting DRM-protected M4P files to other formats:
- Install Soundflower, and in Sound Preferences set the default audio output to Soundflower (2 ch).
- Run Audacity and set the recording device to Soundflower (2 ch), stereo.
- Start Audacity recording.
- Play the M4P file in iTunes, QuickTime Player or qp.
- Stop Audacity recording. Trim silence with Analyze > Silence Finder. Save as Ogg Vorbis, WAV or MP3.