colt45

Are you using 2 music couplers? Does they have a volume control?

(1) R & L are apparently the Right and Left outputs of a stereo signal. If you can, try adjusting the Balance from the Comcast box so you get acceptable levels on both Avaya and Cisco.

A stereo connection is 3 wire, R, L, Com (Gnd). If you connect R&L together you get mono. If still not right, try (2).

(2) If converting to mono does not work, try running the music from Comcast through a 0-5k potentiometer (volume control) then into Cisco. Turn Comcast up until Avaya is happy then use the pot to attenuate the signal toward Cisco. Radio Shack will have the Pot and a cute little app box to mount it in.