What was found
AIRCRAFT WAS GROUNDED: AN EMERGENCY WAS DECLARED AND THE AIRCRAFT RETURNED TO MEM DUE TO COMMUNCATION SINGLE PTT STUCK. CLIMBING THRU FL220 UNABLE TO TRANSMIT ON VHF1, VHF2, VHF3 -(WOULD WORK SHORTLY INTERMITTENT FAULT DID HEARD THEN WOULD STOP. TRANSMITTING.) THE AIRCRAFT LANDED SAFELY AT MEM WITHOUT FURTHER INCIDENT. MAINTENANCE REPLACED THE AUDIO MANAGEMENT UNIT (AMU) PER AMM 23-51-34 AND ACCOMPLISHED A SATISFACTORY OPERATIONAL CHECK.