What was found
AIRCRAFT WAS GROUNDED: CAPTAINS COMMUNICATION COMPLETELY INOPERATIVE USING ANY HEADSET. INTERCOM LOUD BUZZ. ATC-VHF LOUD BUZZ MISSED ATC CALLS INCOMING TWICE. HANDSET COMMUNICATION SEEMS TO WORK FINE. AFTER SWITCHING HEADSETS TO PERSONAL SINGLE PLUG HEADSET OR USING AIRCRAFT TELEX HEADSET, COMMUNICATION IMPROVED. ATC REPORTED STATIC BUT READABLE. TROUBLESHOOT COMM SYST. FOUND BAD P/A AMPLIFIER REPLACED P/A AMPLIFIER IAW AMM 23-31-01. AND OPS CHECKED SYST IAW AMM 23-51-00.