This virtual tag works: $If($Contains(<Path>,"\MusicBee\Music")=T,M,$If($Contains(<Path>,"\Inbox")=T,I,$If($Contains(<Path>,"\Audiobooks")=T,A,$If($Contains(<Path>,"\Podcasts")=T,P,?))))
However, once I save it MusicBee strips out the quote marks, and if I update the virtual tags after that it stops working.