M4P (MPEG-4 Protected) is a digital audio file extension for the AAC (Advanced Audio Codec) container format. It uses lossy compression and is an audio-only file. In most cases, M4P files are purchased on Apple iTunes, which means they are DRM-protected. M4A is the unprotected version of M4P.
AU is an audio format standard in early web pages NEXT systems. It is also called u-law or Sparc-audio format. Typically, AU files can store audio data and annotation blocks. Initially, it was a headerless audio format, but newer versions have a header, thanks to the six unsigned 32.bit words.