Definitely one of those "HDMI cable removal" issues that occur every so often on this laptop. I tried viewing a YouTube video before restarting, and it was dead silent, too.
After a system reboot all is back to normal. One of those "brain farts" because I should have remembered this issue occurring in the past, but it's been ages since I've been routinely using HDMI output from this laptop, so that memory faded (but is fresh again now).
Brian - Windows 10 Pro, 64-Bit, Version 1809, Build 17763
Puritanism: The haunting fear that someone, somewhere, may be happy.
~ H.L. Mencken