Using line input for audio on a Mac, intermittent problems

I’m using zoom on an Apple Macintosh, and I’ve used various audio interfaces with difficulty. The audio device shows up in the audio settings window of zoom, but when I click on it to test it, the meter doesn’t move and I can’t hear the playback of the test.  I switch to an alternate, same story. Only the internal microphone works with no problem.

This happens with a USB audio interface, and with the line input of the Macintosh itself. I go into the mac’s audio midi setup utility, and fiddle around with different sample rate and bit rates, changing the system input to the one I want, and eventually something works but it’s not consistent. Why doesn’t it just work?

The other problem I have with audio is that after I’ve done the audio test and it works, and I join with computer audio, I’m still muted even though I am the host. Am I missing a preference or a menu setting?