Out of everyone I have talked too, including music theorists, performers, conductors, and so on, all of them say there is not a good way to classify all of music in one system (at least not yet).
Musicals, stage shows, operas, and even some popular music may have a lot of different people involved (performer, composer, librettist or lyricist, producer, etc.) and different people care about different things. One person may be interested in all Cole Porter songs, while another is interested in the same mp3 because Ella Fitzgerald performs them. Ricky Ian Gordon's Bright-Eyed Joy is a collection of poems from the likes of Dorothy Parker and Langston Hughes that he set to music, and the songs are performed by well-known concert singers like Audra McDonald, so who gets the place of honor in the mp3 tag?
For now, it is your personal preference.