What was found
(CAN) DURING A MISSION, THE PILOTS SELECTED THE MUTE SWITCH ON THE AUDIO SYSTEM TO TALK AMONGST THEMSELVES. UPON SELECTING THE MUTE SWITCH TO TRANSMIT, THEY FOUND THAT THEY COULD NOT COMMUNICATE WITH TOWER OR OTHER AIRCRAFT. THE PILOT SELECTED THE APPROPRIATE TRANSPONDER CODE TO COMMUNICATE THEIR SITUATION TO SOUTHPORT TOWER. AIRCRAFT RETURNED TO BASE AN LANDED WITHOUT INCIDENT. UPON INSPECTING THE TROUBLE, THE AVIONICS TECH NOTED THAT THE AUDIO MUTE SWITCH HAD FAILED INTERNALLY IN THE MUTE POSITION. THE MUTE SWITCH WAS REPLACED, TESTED SERVICABLE AND THE AIRCRAFT RETURNED TO SERVICE.