What was found
WHEN USING SATCOM 1, TRANSMISSION FROM AIRPLANE TO GROUND WORKS AS ADVERTISED BY PRESSING ACP SATCOM 1 PUSH BUTTON, BUT NEITHER PILOT CAN HEAR ON SATCOM 1 UNLESS SATCOM 2 VOLUME IS SELECTED AND TURNED UP. WHEN USING SATCOM 2 GROUND RECEIVER OVER TRANSMISSION BUT NEITHER PILOT COULD HEAR ANY AUDIO. ACFT GROUNDED. PERFORMED SATCOM COMMUNICATION SYS TEST REF AMM 23-15-00. TESTED BOTH SATCOM NR 1 AND NR 2 FROM BOTH CAPT AND F/O 'S POSITION. OUTGOING AND INCOMING TRANSMISSION NORMAL. ETOPS SIGNIFICANT SYS GROUND VERIFICATION SUCCESSFUL.