Mini M4 with good sound in, but no sound output

I'm running a synth output with ¼" -> USB-A audio cable, then that goes into a USB-A -> USB-C adaptor, then straight into the front of the Mini M4.

Headphones at the synth tell me the synth is playing properly. The Mini shows me a proper varying sound input (the bars jump around as I play the notes). But, none of the various sound outs will work when I go to System settings or use Logic Pro: bluetooth to AirPods, internal Mini speaker, headphone out to an amp, and USB output to NAD amplifier.

So I'm stymied. Looks like the signal gets in there, but why can't anything come out? I've restarted everything, changed USB inputs on the Mini.