No, Volume adjustment (e.g. when tuning the knob) looks good in the display but doesn't change anything. AFAIK even when changing the source to AUX the noise is there. So I would say that the problem ist in the soundpart of the board, no software issues, just the prebooster (or whatever it is named) before letting out the hundred or so Milliwatts to the Cinch plugs.

Maybe I could change a sound IC? Or a broken capacitor? I don't know where I have to put my two left hands .

Rolf