Solved Stereo Mix Problem (Self-Solved)

Windows 11 21H2 (OS Build 22000.527)

I recently installed new USB computer speakers, to replace 3.5mm ones that use Realtek Audio. As I want the living room Realtek speakers to continue to work when listening to the Pebble V3 speakers, I discovered Stereo Mix, and on-line instructions on how to configure it.

My problem is that Sound shows that Pebble V3 is the Default Device, and Speakers/Headphones (Realtek Audio) is ready; when I test Speakers'Headphones, I can hear the sound from the computer room. Clicking on Recording in the Sound module shows that Stereo Mix is the default device, and its Properties sheet shows that Realtek has been selected, and Listen says to listen to this device, which is Speakers/Headphones.

Following the instructions, having figured out how to make the Stereo Mix settings, or at least I seemed to have done so, I restarted the computer. I have audio on the Pebble V3 speakers, but no sound through Speakers/Headphones. One curiosity is that Stereo Mix shows the use of the Realtek black output jack rather than the green jack that I have always used. I plugged the living room cable into the black jack, which did not produce any output, so I moved the cable to the green jack, and still have no music.

The Device Manager for Audio inputs and outputs lists Speakers (Pebble V3), Speakers/Headphones (Realtek Audio), and Stereo Mix (Realtek Audio), all of which are working properly.

Why is this not working? Have I overlooked something or done something improperly?

====
This morning, (19Feb22), I found out what I had done improperly, which was to change the default device from the Pebble speakers to Realtek Audio, and then use Stereo Mix to use the speakers. Please do not ask me why making that simple change fixed the problem; all I know is that when I turned on Internet Music (that is what I titled that function in the AV remote) in the living room, I was listening to the same Bach Triple Concerto that I am listening to as I write this in the computer room.