MP4 and M4A File Support
M4A files are simply MP4 files ([MPEG-4 Part 14 container files http://en.wikipedia.org/wiki/Mp4]) that contain only an audio stream (usually AAC encoded). There is no difference between an M4A file and a MP4 file other than the file extension. In fact, the M4A extension was actually not part of the original MP4 "specification" but was invented by Apple to differentiate audio-only MP4 files from video ones.
In order to play these files back you need to either have Quicktime Player installed, or you need to install the proper DirectShow filters and enable DirectShow M4A support. Generally, DirectShow playback will give better results for M4A files, but because most people have Quicktime installed already, MC disables DirectShow support in favor of Quicktime by default.