The MIDI piano sound is currently set to a default 'piano' sound only, it would be great if users could select their own sounds from a palette of presets, or the sound would be switched according to the song style you play (e.g. when you're playing accompaniment, it changes to a string ensemble, for example)

It's always possible to connect an external synth, in which case Yousician can send the appropriate MIDI Program Change messages to select the proper sound which is bundled with the song.