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.

Reproducible: Always

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.

Actual Results:  
No sound was played

Expected Results:  
The selected sound should be played