No sound on note input in parts after save/reload

This is due to the same underlying cause as #278918: MIDI export of a part crashes - parts don't have their own playback, they just rely on the master score, and while things are set up OK when the parts are first generated, it isn't set up again on load. So:

1) default score
2) enter some notes
3) generate parts
4) visit the part, click some notes to verify they play
5) save
6) close
7) reload
8) visit the part again and try clicking some notes

Result: no sound at all. Hitting spacebar to commence playback works, because as noted here, it's really playing the master score. But the part has no sound at all at this point. Entering new notes is similarly silent.