Quoted from ravve:

I did not check aux out since it seems too complicated/time consuming to me, but will do it when I have no ideas left
Today I measured some voltages and noticed that the +5v line that comes from J3 seems to be where I should dig into. I measured it at C79.
It measures 4.84 VDC when started from cold state, and slowy increase until it reaches 4.91 VDC within a minute or two, and 4.93 VDC if waiting longer.
Compared with another working machine, I got 4.915.0VDC within 30 seconds, which makes me beleive that my issue isn't there (since it takes more than 30 seconds for the audio to become good).

Quoted from ravve:

Well thank u, I suppose u mean C18 & C19, not C29?
I also am a bit afraid to remove the big caps, to not damage traces.
Do/could C20/C21 has any effect on the voltage on C79?
If yes, then I def.wiill change them as next step. If no, I think my issue lies somewhere else?