Sorry for mentioning LibreElec. CoreElec is great stuff, but sadly not supporting my box anymore. I plan to stick with CoreElec 9.2.8 as long as possible (Never change a running system). But when it does not work anymore I will try LibreElec 11.0 (Alpha), before dumping the box.
That sound sensible. But what should it be? The sound over optical works if I boot the original Android. So there should neither be a hardware problem on the box, nor on the optical cable, nor on my audio receiver.
The volume of the box was down. I never set the volume on the box. In my setup the Logitech Harmony remote control always sets the volume of my amplifier. I do not know how it could happen that the box volume was down.
Sorry for bothering you with this and thanks for all the comments.