You can update audio drivers to see if it can fix Stereo Mix not showing in Windows 10 issue. If you still don’t see Stereo Mix in your Windows 10 computer, it might because your sound card drivers are outdated or the new sound card driver doesn’t support Stereo Mix feature. You can then follow the guide above to enable it. After this, the Stereo Mix device should appear in the list of recording devices. Check how to make Realtek HD Audio Stereo Mix feature visible. If you don’t see Stereo Mix under Recording tab in Sound window, it might be hidden in your sound card.
Read More Fix Realtek Stereo Mix Not Showing or Missing in Windows 10