It seems I need to use jack for this. But, I don't know yet if jack can do what I want.

On Fri, Mar 29, 2019 at 8:25 AM crocket <crockabiscuit@gmail.com> wrote:
My Raspberry Pi can turn audio gears on and off via relays.
When my desktop starts using its speakers, I want it to tell Raspberry Pi to turn on the speakers via a relay.
When nothing has used the speakers for a minute, I want it to tell Raspberry Pi to turn off the speakers.

I can do the same for a USB microphone. A USB microphone can be turned on and off by uhubctl.

Does ALSA have a callback API that I hook into? Does jack have it?