Working on a desktop with Win 10 and Audacity 3.0.4
I have a website streaming and would like to record from it. I’ve enabled Stereo Mix, but seem to be missing something here. If I click on the bar at the top right to monitor, it doesn’t show anything. If I try to record, I’ve got silence. I have tried with different websites as well but same thing. Guessing I’m doing something idiotic here, but totally clueless…

Have you tried WASAPI loopback recording of Speakers/Headphones (loopback) ?
If I select WASAPI and choose my headset then it does look like it will work. Does that provide the same quality of recording as it would do without the loopback method?
Does that provide the same quality of recording as it would do without the loopback method?
Maybe better. 
I’d guess Stereo Mix doesn’t work because you are playing through your Plantronics headset (USB?) but recording the (silent?) Stereo Mix from your Realtek soundcard.
Thanks for including the screenshot!!!