MC has currently these MPEG file associations (besides MP3 and the MP4 variants):
mpa, mpga, mp2, mpeg, mpg, m1v, mpv, mpe, m2v
I have Steinberg's Wavelab 5 (a pro audio editor). It has an MPEG-1 Layer 2 encoder.
The offered file extension options are: mp2, mpa, mpg, mus
I remember now that I have previously changed the Media Type of my MP2 audio files because MC imported them as video. Afterall, I think the best system would be this:
mpg, mpeg, m1v, m2v, mpe, mpv, m2p, mpv2, mp2v, mpeg2 => Video
mp2, mpa, mpga, mp1, mp2a => Audio
mus is also used by the well-known
Finale notation software and I have never seen a .mus MPEG audio file so I guess it is not needed.
EDIT: Added m2p, it is used by Canopus for MPEG2 videos.
EDIT2: Added more extensions. The oldest MPEG-1 layer 1 audio format uses mp1. Sometimes the mp2a (MPEG-1 layer 2 audio), mpv2, mp2v and mpeg2 (MPEG-2 video) extensions are used too.