With shock and horror, I realized that my computer, an M1 Mac, could not play MIDI files! It's been a while, to say the least, and some cursory research suggests Quicktime's support for MIDI died when it went 64-bit. I set about finding a simple, no-nonsense player and found Peter Wunder's MinimalMIDIPlayer at github, which is free of charge and worked perfectly. It even has soundfont support! The others I googled my way to either cost money or would not launch on my machine.