It should play on first channel that nothing happens at the moment, if the music is in effect it should detect empty channels by looking which channels are "selected" in music pattern editor to play.

Actually a good suggestion for once. I think we'd benefit a ton from a system that only temporarily clobbers music like gameboy games tend to have (think pokemon), on a chosen channel. Right now playing sfx over a music channel breaks the music forever, which kills the harmony in We Missed You!

You should just fork PICO-8 at this point.

Pizza, if it would be open-source, believe me - I would.

@Rhys: This was actually quite common in the 8bit era, not only on gameboy, but on NES, C64 and other 8bit systems as well. Just listen closely when you'll be playing Contra or whatnot next time.