Playing Piano Midi Notes on an Original Espruino Board using the DAC

This looks great! Do you have a recording of it in action anywhere?

The tempo of the notes increases with pitch. How to solve this?

You could have one sample per octave?

With only 2 DACs, polyphony is limited. Using PWM can produce synthesized sounds.

Waveform itself can do polyphony on one DAC channel! Give it a try - just start a few Waveforms on exactly the same output :)

You also mentioned MIDI... I'd love to be able to import notes from a MIDI file into Espruino to play - it might help to make up for my lack of musical talent!