Hello, again. Since I had to work for a long time, I could not reply until today.
Unluckily the alsamixer does not change anything at all.
But when I plug in a mobile loudspeaker into the headphones connector there is stereo sound.
But the internal loudspeakers only work on the right channel.
When using Windows 10 the soundcard provides sound on both channels, so a hardware failure is clearly not present.
The search continues ... but thanks a lot for giving me some advice!