On the PA-28-181 that I fly, there's a circuit breaker called Audio/Mkr. What is that? In the aircraft, there's also a dual GNS430 if that makes it any more specific. I was thinking it could be controlling the speaker on the aircraft?
This circuit breaker is responsible for your Audio Control Panel. It's called Audio/Mkr. because the Audio Control Panel in most aircraft includes the Marker receiver and displays (maybe you noticed the three colored lamps on the left side reading O, I and M, colored white, blue and orange).
If the circuit breaker is out you loose your audio on the one hand side, on the other side you also loose your ability to identify Marker Beacons (functionality explained by Carlo).
It's the circuit breaker, most likely, for power from the avionics master bus to the audio panel as well as the marker beacon receiver.