Did you have a look at this thread:
It looks as though there are many more types for mediatype:
Movie, Normal, Audiobook, Music Video, Short Film, TV Show, Ringtone
And: these categories are only valid for MP4 files anyway (which in a way has been indicated by you as you want to delete the field from mp3s)
You could create an action of the type "Format value" for MEDIATYPE
Format string: $ifgreater($strstr(%_tag%,MP4),0,Music Video,)