It is likely either a power supply or mute control problem somewhere - or a corrupted parameter?
Have you done a "Factory reset" on it, to ensure it's not something messed up in its settings?
Check the "Mute" circuit - in the right hand side of the output board diagram on page 14 of the service manual.
If the 12V supply is too low or something has failed in that area, it will be turning on several transistors that short out the audio signal, to prevent unwanted noises as it switches on or off.
Check the voltage on R3 / R4 / R10 / R11. Those should be near 0V when it's supposed to be working normally. 0.6V or higher means the mute circuit is active, killing the audio.
With adequate power, TR6 should be on, pulling R12 low and turning off the digital transistor (a transistor with a built-in series resistor to the base & a base-emitter resistor. Everything from the output of TR7 to the base of TR5 should be close to 9V.
I've grabbed the relevant part of the diagram, below.
Other than that, the two channels are totally separate from the DAC through to the output sockets. See IC10 on the main board (DAC) then IC11, the first audio stage.
Full service manual here, if you do not have it: