When a KNotification event with sound is triggered from an application, the sound will play once but when the same event is triggered later then it's muted until the application restarts.
I'm not sure phonon-vlc is the only one to blame here but I narrowed it down to commit 3d0a13dc0ee0cdd361920c8680b839bc2f934d07.
The problem didn't exist before that, and it doesn't exist in the gstreamer backend neither.

Reproducible: Always

Steps to Reproduce:
1. Open dolphin.
2. Right click on a file, choose delete (not just move to trash bin). A warning sound is played.
3. When asked for confirmation, cancel.
4. Right click on a file, choose delete.

Actual Results:  
No sound is played.

Expected Results:  
The warning sound should be played.