Hi!
You probably didn’t set your multi-timbral instrument’s MIDI track(s) to automatic solo mode! Either Alt-click the track’s solo button or press Alt+S when it’s selected (please ignore the outdated shortcut key combination shown in the video).
Normally, all other tracks are muted when you bounce a track. But as you can see, if the MIDI track is muted, no notes get passed to the instrument, and so the bounce output is silent!
Hope this helps. 🙂