As a follow-up note on my own experience, I had not realized that when you plug in the HDMI cable Windows automatically switches audio output to the device associated with HDMI (which, of course, it has to do) but that it sometimes does not automatically switch it back to speakers/headphones when the HDMI cable is withdrawn.
I stumbled upon this by accident when I used HDMI again after this last incident. If I go to the volume control in the system tray and open it, it was still showing the device as HDMI even after the cable was removed. All I had to do was hit the dropdown for the device in volume control and switch it back to speakers/headphones and all was well. This would not, however, be something simple for a screen reader user to do if the only output they had would be through the speakers/headphones, as you'd have no speech output to get you there. Rebooting without the HDMI cable inserted makes the audio automatically redirect to an available device, in this case the speakers/headphones.
Brian - Windows 10 Pro, 64-Bit, Version 1809, Build 17763
Puritanism: The haunting fear that someone, somewhere, may be happy.
~ H.L. Mencken