Version:            (using KDE 4.2.0)
OS:                Linux
Installed from:    Debian testing/unstable Packages

This is what KHelpCenter says:

Change sound player options
If you click the button labeled Player settings, a small dialog box will appear with three radio buttons:
Use the KDE sound system
If a mark is in this radio button, KDE will play all sound notifications through the KDE sound system (aRts). You can adjust the volume of system notifications using the slider.
Note
The slider only affects KDE system notifications. Changes to this slider will not affect other sounds on your computer.


I'd like to keep my laptop's hardware mixer volume to the fullest and keep KDE System Notifications to like 20%.
Since I use PulseAudio, I will be able to tune individual apps with higher audio volume.

The problem is that KDE System Notification's Volume slider is not effective. No matter how much I slide it, the volume is still the same.