Comment # 4 on bug 922401 from Wolfgang Bauer

PS, meanwhile I think that this is actually causing your problem:
(In reply to Wolfgang Bauer from comment #3)
> Maybe you raised the volume for notifications in PulseAudio itself?
> PulseAudio basically allows a separate volume setting for each application.
> You should be able to set the volume for notifications in KMix or
> pavucontrol.

If you click on KMix's icon in the system tray, you should see _two_ volume
controls. The left one is the general volume, whereas the right one is the
volume for notifications (although it has the same label).
If the right one is set to (nearly) 100%, this would exactly cause what you
describe: notifications would be played at 100% volume, and also crank up the
overall volume to 100% (because of PulseAudio's flat-volumes).

So move the right slider to a lower value, about 25% maybe, and your problem
should be "solved".

If that's the case, the only (KDE) bug I can see here is the incorrect
labelling of the right slider (which can of course be confusing), probably
caused by openSUSE's patches. But then, we should be able to drop the patches
completely anyway next month, because this should be fixed upstream.