I’d love it if a group could collude on a standard for music signals.

Imagine this: You have a music player following this signal standard.

Game starts, it signals GAME_STARTED, and the media player signals STOP_GAME_MUSIC, so the game itself plays no BGM, leaving it to the music player. But, then the game can also signal later on: THEME_MENUS, THEME_EXPLORE, THEME_COMBAT, THEME_BOSS; and the media player can respond to that by cross fading between playlists built for each.