M4P is one of extensions for MPEG4-AAC compressed audio format developed with the cooperation and contributions of several companies including Dolby, Fraunhofer IIS, AT&T, Sony and Nokia.
This format was officially declared an international standard by the Moving Pictures Experts Group in April 1997.

Currently M4P (MPEG4-AAC) is default audio format of Apple's iTunes, iPod, iPhone and the format used for all iTunes Store audio.

TuneCab Media Converter can convert M4P to MP3, AAC or WAV in case if your music file was legally obtained and can be played on your PC by QuickTime Player.