Wow!
I had opened the Control Panel → Hardware & Sound → Manage Audio Devices window earlier but couldn’t find anything that looked like it would help. I was getting ready to shut everything down and turn off the computer for the night and browsed around again for a bit. The Playback tab showed only 1 item - Realtek Speakers. The Recording tab also showed only one item, the Realtek microphone. I just happened to right click within the window which showed some options which included:
an unchecked “show disabled devices”. I checked it and “Stereo Mix” was one of the available, but disabled devices. It was there all the time, but just hidden from view. I clicked the option to enable it, and presto, it now shows up both there, and in Audacity. See attached screenshot.
Awesome!
