Hi Daniel, looks like it was a problem with Windows 10. For future readers, Windows 10’s built-in MIDI system has a history of malfunction when sound output devices are set to “Exclusive” mode. Fix this by going into the Control Panel, selecting Sound, choosing the Playback tab, clicking Properties for each output device, choosing the Advanced tab, and de-selecting “Allow applications to take exclusive control of this device.”
Daniel, now I have a new question. I can’t seem to select a GM patch (or any program number) for a General MIDI instrument. In Play mode, I see a drop-down for setting the device’s channel, which works fine. The pencil icon for editing the GM instrument under “MIDI Instruments” (in the right pane) doesn’t do anything (oddly, neither does the trash-can icon), and I can’t find other options anywhere.
By the way, you probably don’t remember, but you and I had a marvelous correspondence about a fringe music notation issue I had about six years ago, and you were an immense help to me. I’ve followed “Making Notes” since its creation, have enjoyed reading every post in full, and am purchasing copies of Dorico for my new institution this year!