M4A or M4P or any other Apple format.
[nerdmode]
M4A is not an Apple format. It is simply a MP4 file which Apple renamed M4A to denote audio-only files. Same goes with M4P. M4P is just a MP4 where the internal audio or video streams are encrypted with FairPlay DRM. Technically, there is no difference between MP4, M4A, M4V, M4P, M4B, or M4R files. They're all just MP4 files.
While the MP4 file format was based, in part, on Apple's MOV container, it is an open standard controlled by international standards bodies.
In fact, while most people think of AAC (the codec used in most of those Apple M4A files) as an "Apple" compression scheme, it actually has very little to do with them. It was actually developed by AT&T Bell Laboratories, Fraunhofer IIS, Dolby Laboratories, Sony Corporation and Nokia, and is part of the MPEG-4 standard. Apple just happened to select AAC as the codec used for songs sold on iTunes, and wrapped them in MP4 containers. They used the standards.
[/nerdmode]