Comment # 1 on bug 922401 from Wolfgang Bauer

(In reply to Christian Bachmaier from comment #0)
> System sounds, e.g. KDE system start are too loud. After setting the
> loudness of systems sounds (during playing the sound from system settings'
> notifiations panel to 50%) in kmix icon, it is automatically restored to
> 100%. That means, playing it anew is too loud again. As far I can remember
> this "feature" was already present in 13.1 .
> 
> I am not the only one:
> https://forums.opensuse.org/showthread.php/492575-66-sound-volume

Hm?
That thread is 2 years old and actually about the VolumeOverdrive feature, i.e.
that PulseAudio/pavucontrol allow to crank up the volume to 153%, but this is
shown by KMix as 100%. So PA's 100% volume would be shown as 66% by KMix.

> Looks like an upstream bug:
> https://bugs.kde.org/show_bug.cgi?id=324975

Yes. But openSUSE has a patch for this since 13.1.
Are you really using the package kdebase4-runtime from the distribution?

> Any workaround?

Yes, several:
- turn off PulseAudio (YaST->Hardware->Sound->Other->PulseAudio Configuration)
or uninstall it
- set "flat-volumes = no" in /etc/pulse/daemon.conf, this will prevent any
application from changing the overall volume
- you could also set a lower volume in the notifications settings, but that is
removed/disabled by openSUSE's patch, but have a look into 
~/.kde4/share/config/knotifyrc and modify the "Volume=" setting to a lower
value (I'm not sure whether this has any effect though)