New
#11
The generic, basic function, Windows installed High Definition Audio device drivers DOES NOT have Stereo Mix.
With some of the later Win 10 versions they have "depreciated" (deleted support) some older hardware. I would guess it was OK on an older version and with a later version it no longer supported whatever driver you had and Windows installed the HD Audio driver.
Is that disabled?
If not, Use Virtual Audio Cable or VoiceMeeter.
Virtual Audio Cable
VoiceMeeter
My System is 20H2 and Stereo Mix is not missing