Yes, I think I can see the audio settings page with check-boxes that you refer to, and I can get there by different routes: Menu/Tools/Audio Settings... ; Menu/Tools/Option.../Audio ; Option/Options/Audio ; Option/Audio Options ; but all these lead to the same place.
Once there, I have check-boxes: Microphone ; Speakers ; Mouse Clicks ; Only show db levels during recording. I have all except Mouse Clicks checked.
Next to Microphone, I have only one option: Microphone (Realtek(R) Audio); and next to that there is System Mixer... which seems to say the Realtek is my default (and only) device.
Next to Speakers, I have the choice of MMDevice and Filter Driver. I don't know what these are. I've tried both, but can't see (or hear) any difference. It also doesn't affect what I see when I press the System Mixer... button. In both cases there are two columns, Device Speakers and Applications System Sounds, with volume control sliders both at 100%.
With these settings, I can record my screen, and the audio is there, but it's so low as to be almost unintelligible. Moving the sliders away from 100% makes it worse.
So, is the audio setting page I've described the one you meant? And, are the sliders under System Mixer... what you meant by StereoMix? If so, I gather you can make your setup work adequately with the sliders not at 100%. If that's the case, then I guess I've got to look elsewhere.
In any case, thank you again, for your time and interest.