MPG-4 is not an audio format or filename extension, but probably you mean the .mp4 filename extension. There is also the MPEG-4 standard, but MPG-4 is not a single format. It is a standard that defines several things. Wikipedia provides some information about MPEG-4:
http://en.wikipedia.org/wiki/MPEG-4In general, MC works better if the MP4 audio files have the extension .m4a. This extension was introduced by Apple and it has gained wide support. It makes possible to easily distinguish the MP4 video and MP4 audio files from each other.
MC may have imported your files as video (if they are .mp4 files) and they might be under the video gategory. For importing and playing any .mp4 or .m4a files MC needs some installed OS level codecs which are not a part of a standard Windows installation.
If Apple's QuickTime is installed (either the stand-alone version or as a part of iTunes) MC should be able to import and play MP4 files.
However, MC provides an alternative method for playing MP4 files through DirectShow. The DirectShow playback method is usually preferred because it provides more configuration options, better audio or video quality with certain types of MP4 media files and makes possible to use MC's internal audio engine with .m4a files.
More info:
http://wiki.jrmediacenter.com/index.php/MP4_and_M4A_File_Supporthttp://wiki.jrmediacenter.com/index.php/DirectShow_Playback_Guide... and in various threads in this forum.
EDIT
If the files are DRM protected .m4p files from Apple's iTunes store the situation may be different. It has been possible to import and play them when QuickTime is installed on the PC and the user rights are properly registered with iTunes, but I don't know if Apple has made any changes that could affect this. I don't have any DRM protected files. Perhaps other forum users know more about this.
