Radio works, CD works, No sound. MOST Bus?

Hello!
I have a problem with my sound system
Audi A6 2.4 2005 year model
MMI Basic Plus
The radio / MMI / CD works but i get no sound from the speakers.

Sometimes when i start the MMI the screen lights up and then goes black and starts again.
I can play cds, i can see the seconds go up.

I have read a bit about the MOST Bus.
I disconnected the optical cable going to the amp
When i turn the ignition on i can see a red light from the optical cable but after a couple of seconds it flashes and then goes out.

Is the red light supposed to light up all the time?
I suspect that their might be a glitch in the optics so thats why it flashes and then goes out. What i have read so far is that the amplifier is the last unit in the ring?
So the next step i thought would be to disconnect CD changer and see how the optics behave when it is disconnected.

This fault has been for 2 years, the workshop hasnt been able to find the problem.
Kind regards
Kevin