The system's default notification sounds (including 'im' and 'k3b' groups, etc.), when referenced by their path (i.e. /usr/share/sounds/KDE-[...].ogg), do not play - either through use of the appropriate setting button or when a notification generated).
However, simply referencing the file name alone (i.e. KDE-[...].ogg) results in the sound playing correctly - both through the configuration module and through notification generation.
Steps to Reproduce:
1. Open 'Notifications' configuration module
2. Check 'Play a sound' for any notification type
3. Enter a path to a system sound (i.e. /usr/share/sounds/KDE-Sys-App-Message.ogg) - This also applies if selected from the file dialogue
4. Try to play the sound by clicking on the play button to the side and/or by having a notification generated for the associated event.
No sound was played
The selected sound should be played
I tested at my side(v5.6.90), the sound play out. So could you retest in your side again?
If it still fail, does the ogg file play nicely if you open it in file manager directly?
*** This bug has been marked as a duplicate of bug 407397 ***