I have Stereo Mix working now. It probably was working for quite a while without my being aware of it.
What I discovered, by accident, was that if I switched to Stereo Mix, nothing happened in TM. BUT when I had closed the program and then restarted it, it now worked. In other words, it accepted the input set-up that existed when it started, and changes I was making to the sound input selection after it opened were not transmitted to my software.
[perhaps I would need to rewrite something, but I don't care]
This was in contrast to the way the program had always performed under W98, and XP. (I have someone else's old W7 machine, and that in itself has some strange behavior). Then, when I changed the Recording Control from say Stereo Mix to say Microphone, the input to TM changed on the fly.
So I can make it work by assuring the setting I want is operative when I open TM.
But with this advance comes the realization that W10 has the same stupid, irritating function that W7 exhibits (now reminded of that by again using this W7 machine too) - namely, that the recording volume is directly influenced by the output volume! Stupid. In other words, I need to have the speaker output at full volume in order to get a decent input level. So I have to turn off my speakers if e.g. I want to record a streaming concert on Audacity (which BTW has its own input control that does allow Stereo Mix, or microphone, or Line In to be changed while the program is running) but practice my harpsichord, I have to silence my speakers. In XP I could mute the output via the little speaker icon, and nothing would change in the volume of the recording.
If you know a way around this, please let me know! We go from stupid to stupider.
I knew Stereo Mix was possible, as using Blueberry free Screen Recorder to make videos of Zoom things I might not watch live, it had a choice to record microphone, or "computer sounds", i.e. Stereo Mix.
I realize, writing this, I need to experiment whether their recording level is influenced by the output (speaker) level.
Well, thanks for helping patiently. Whatever driver I installed most recently, that worked ----- only I didn't know it.