when the user wants to set a sound for a notification, it will not work.

Reproducible: Always

Steps to Reproduce:
1. launch kcmshell5 kcmnotify
2. now set an notification sound for a specific action
3. try to preview this sound (or trigger the action)

Actual Results:  
no sound will be played

Expected Results:  
sound should be played

as workaround 
https://forum.manjaro.org/t/i-cant-use-custom-sounds-for-notifications-in-kde-plasma/4633/3

we found that if we replace the path to an sound file (for example) 
/usr/share/sounds/KDE-Sys-App-Message.ogg
with "file://" in front:
file://usr/share/sounds/KDE-Sys-App-Message.ogg
the preview works and the notification sound also works.