not quite true what I’ve written in the subject, actually the volume of the external speakers is adjustable, but only via the Volume Control GUI under ‘Playback’. There I find the used mediaplayer listed and under it there’s a possibility to adjust the volume and it works.
However the volume of the external speakers is not adjustable via the loudspeaker symbol on the panel and also not via the volume keys on the keyboard. Pressing them will display an appropriate visual feedback, however the volume does not change.
Volume control for internal speakers works fine.