I have a microphone I want to keep available for only one single app (i.e., other apps cannot even see the microphone besides the one I select).
I've searched everywhere and all people say is "disable the microphone," but I want it always active, always unmuted, and always selectable in the one app I want, and have it hidden from every other app.
How can I hide my microphone from all apps other than the one I select?
If your application supports ASIO, disable the microphone in Windows (in the Sound Control Panel aka mmsys.cpl via Win+R, go to the Recording tab, right-click your microphone, and select Disable) and install an ASIO Router like ASIO4ALL (free) or VB-Audio Matrix. These tools can see raw hardware streams even when they are hidden from the standard Windows Audio wrapper.
If your application doesn't supports ASIO, try VB-CABLE Virtual Audio.