Thanks for your quick answers.

I will first contact the seller as we can fix the problem.
If that does not help, I'll buy a new speaker.

Hello
I installed the new firmware and made a factory reset.
Nothing has changed.

In this video the problem starts from 1:37 https://www.youtube.com/watch?v=Ot_11RuNU7g
And in this video the problem starts from 3:17 https://www.youtube.com/watch?v=mzB-0PkRIG4

With my BV 8000 the volume control did not work. No matter if videos from Youtube, Facebook above the browser, the volume is constantly changing.
The "App Sound" app does not help. What else can I try?