Comment # 6 on bug 922401 from Wolfgang Bauer

(In reply to Christian Bachmaier from comment #5)
> I see 2 sliders. No matter which one (or both) I move to say 25%, playing
> the next system sound in KDE control panel or e.g. automatically at login
> time resets (automatically) both to 66% again. This cannot be intended.

IIRC, playing a system sound in the KDE control panel will always play it at
100%.
So don't use that to verify the volume.

But actual notifications should be played at the correct (lower) volume.