i don’t think picomite/mmbasic has that functionality or indeed other than sampled audio as you pointed out, a way to write it yourself
it is possible hardware-wise though, as seen by my sound module in the Lua firmware (you can also just try out the “piano” example it comes with)
if you decide to use it, i’m happy to help and receive feedback on functionality as the interface at the moment is admittedly less than ideal and the firmware really needs some more work