This is a little shell script, that does what should be easily possible in KDEs systemsettings.
It updates your config files to disable all notification sounds!
It is best used, before setting up any notifications, as it will overwrite any notification settings with [system default] - sound.
I tested it with KUbuntu KUbuntu 15.10 / KDE Frameworks 5.
It should work with other distributions and might work with older Versions of KDE. You might have to change some variables though (see comments in the script).
Instructions for dummies:
download to /tmp
$ sh /tmp/silenceKDE.sh