Comment # 6 on bug 1166496 from Wolfgang Bauer

(In reply to Josef Wolf from comment #5)
> > You mean, it does not occur with GNOME + gdm I suppose.
> 
> It does not happen with kde/plasma + GDM
Well, that would actually confirm that it's not a bug in Plasma it seems.

I'd suggest you use gdm then, if the problem annoys you.

> > You'll notice something similar if you e.g. play some music in
> > the session, as somebody mentioned in your mailing list thread.
> > If you press Ctrl+Alt+Fx, the music stops until you switch back.
> 
> This is actually not true. I can hear sound from youtube videos while I am
> switched to a different terminal.
I cannot, also sound from youtube videos is stopped if I switch to another VT.

I'm currently using kdm (from KDE4) here btw.

Still, you wrote:
> I can see the Problem with opensuse-Leap-15.1 as well as with ubuntu-18.04
> (when kubuntu-desktop is installed)
...
> - Problem does appear with sddm, lightdm and kdm

That means IMHO, that it is a more general problem, not just a bug in one
displaymanager (and not specific to openSUSE anyway).