I've just published https://github.com/cmsj/HotMic/ which contains a very good amount of a macOS app I had hoped to complete and sell for a couple of bucks on the Mac App Store.
However, I failed to get it working, primarily because I don't know enough of CoreAudio to get it working, and because I burned almost all of the time I had to write the app, fighting with things that, it turns out, were never going to work.
So, chalk that one up to experience I guess. Maybe the next person who has this idea will find my repo and spend their allotted time getting it to work :)
For the curious, the app's purpose was to be a Play Through mechanism for OS X. What is a Play Through app? It means the app reads audio from one device (e.g. a microphone or a Line In port) and writes it out to a different device (e.g. your normal speakers). This lets you use your Mac as a very limited audio mixer. I want it so the Line Out from my PC can be connected to my iMac - then all of my computer audio comes out of one set of speakers with one keyboard volume control setup.
For the super curious, I'd be happy to get back to working on the app if someone who knows more about Core Audio than I do, wants to get involved!