Sorry to bother you all.
I unplugged the splitter (where 2 connectors are connected to 1) from the Mac Mini. At that very moment the music started playing, through the computer inner speaker.
That made me wonder... Did I connect the correct plugs in the splitter? After some investigation I found there were 3 connectors behind the cabinet. I must have plugged the wrong one in the splitter.
Now I remember my son sometimes goes on investigation. Maybe he had unplugged the 2 connectors. When I figured out there was no N.Radio, I plugged in both connectors, without knowing one of them was not correct.
So finally problem solved.