"Why can't this support import from MIDI like every other piece of software?!?" my musician says, candidly.

In all seriousness, it would be nice to get some kind of import system for music.

That would indeed be great.

Perhaps import of other tracker formats would be the easiest given the target format (tracker).

Agreed, it would be great if MIDI could be imported directly into the cart like a spritesheet. Having said that—it would be pretty easy to hack together an importer.

This (e.g.) could be abused to convert a MIDI file to a sequence of notes as text in Lua array format. You could then use the code in this thread to set the notes directly, and then CSTORE them into the cart permanently.

Some caveats: the Pico8 can only produce notes in the range C2 to Eb7 (+/- an octave depending on the instrument you use). Also I think you would have to subtract 36 from MIDI note numbers to correspond to Pico8 notes, assuming MIDI C4 is 60 while Pico8 C4 is 24.