A file with the M4P file extension is an iTunes Audio file or sometimes called an iTunes Music Store Audio file. It’s really just an AAC file that’s copy protected using a proprietary DRM technology created by Apple.
M4P files are seen when downloading music from the iTunes Store. It can be opened with Apple’s iTunes. However, the computer you’re using iTunes on must be authorized in order to play the M4P file, which is done by logging into iTunes under the same account that was used to download the audio file.
If you want to convert the M4P to MP3, you should use DRM Remover tool to remove the DRM and then do the conversion job.