Thanks Omar_A1,
I had read that article, but since it used language like "some Macs," I didn't feel certain that it applied to mine.
I tried attaching several different wireless audio receivers. All of them registered as headphones instead of audio sources.
Since posting my question, I have found several other places where someone has said you need a "trrs" connector instead of a "trs" connector (apparently it has extra rings on the connector allowing for the computer to separate in from out.
I also found some posting(s) that said in order to use usb-c, you need something with a capture card built in. Without that, it won't recognize it as a source, just a headphone port. I ended up buying something with a capture card.
That seems to have solved the problem of recognizing an audio source. The issue now is... you can't listen to that source using the computer's speakers or a pair of headphones. I ended up recording the incoming audio in voice memo and was able to hear it on playback. I haven't had time to see if there is a way to hear the incoming source without using an app. At the least, I went to the sound settings and made sure nothing was muted and that the output was set to the correct setting. I could see under input that it was registering the audio.
I don't feel the need to solve the loop through issue any time soon, but if you happen to know of a way, I'll look out for a response.
I have Big Sur 11.1
Thanks for your help.