What is the meaing of the circuit breaker marking "Audio/Mkr" in a PA-28-181?
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).