Mission Lover with unusual Mission Cyrus III Problem

[MattvonMatt]

Hello!
First posting here, try to learn.
I have a Mission Cyrus Amp here. Later Model i think, no jumpers for PSX-R mode.
The Issue:
Amp switches on, plays the LED Sequence, all Buttons working inkl wheel are working but... no Sound.
Only a small „plop“ wenn turning the volume knob one "led-step". „plop“ too @ mute button. Mute button tested, fine.
Somebody know these issue?

Thanks for help!