The extension stands for "MPEG-4 Protected". It is an AAC-encoded audio file that includes FairPlay Digital Rights Management (DRM) , which restricts playback to authorized Apple devices and prevents simple file sharing. The Lilo & Stitch Soundtrack in M4P Format

Do you still have an old iPod with orphaned M4P files? Or did you manage to convert your 2005 iTunes purchases before the DRM apocalypse? Drop a comment below.