- a button in the interface (and/or shortcut key/pianokey) to toggle the MIDI volume to mute and back, or
- separate volume sliders for MIDI volume and demo piano
I think the second suggestion (have separate sliders for MIDI input and piano demo) would be the most elegant.
If you have a reliable way of sending MIDI to my piano to have it play the demo with its built in sounds - even better. But that sounds like a nice to have.