Long time ago back when one would be building repeaters with the GM300/M130/M120 radios, always had to move J551 from its default (A) position to the other side (B). But this will of course only change the type of audio available at pin 11, and should not cause to loose audio completely which seems to be the case here. As others have mentioned, you should check if that jumper is installed correctly, or even be present. I did once in a while come across a radio where it was missing altogether as someone decided it was not needed, or who knows what happened. Also make sure that audio is actually reaching the B side.
