Title says it all.
A very interesting request/challenge, @urassari!
It made me investigate a bit into music on the micro:bit using MakeCode.
I did not find out what the channel limitations are for the micro:bit, if any, compared to the Arcade hardware/simulator, discovering that the micro:bit seems pretty advanced with recording and playback capabilities(sampling?).
However, with the built-in MakeCode music library it seems similar to the old one for Arcade, before it got the music composer you are requesting. I find the music.play() function, but not the music.crateSong() functions and support for assets, where I was hoping there was I way to compose the music in Arcade and then transfer to a program and play back on the micro:bit.
No such luck, but I am sure experts in this field like @UnsignedArduino, @kwx etc. may have a much better insight into this than me, if you are lucky enough to tickle their attention enough to reply to this post.
You may like to take a look at the new FlexFX extension that lets you compose new tunes, and even design new instruments to play them onπ