New M1 Mac mini - No audio on USB after sleep

The Exclamation mark in a speech bubble is not an indication of an error, but shows which audio output is configured for system alerts and sound effects.

Having said that, I've experienced problems with external audio after sleep on my 2018 Mini. I suspect it's a bug. I have to kill the coreaudiod process to get things working again.

sudo killall coreaudiod

Then enter an admin password.

If you are changing the Sample rate to get things working, then it may be that doing that restarts coreaudiod. (Assuming it's the same issue.)

Note that some audio apps may crash or get confused if you change the sample rate while they are running.