If headphones are working, the problem is most likely not in the main unit, because the headphone amplifier gets
its signal from the same output that goes directly to the Powerlink sockets.
If the speakers are daisy-chained, it could be a bad cable.
Else:
Check with a scope on pin 5 in the Powerlink sockets, if the audio signal is present.
If not;
Check if 5V is present at 2TR35 emitter, and check the PL-mute signal at its collector.
Check if +/- 15V is present on the OpAmps 2IC5 / 2IC6 / 2IC7 / 2IC8.
Martin