Losing KDE system notification sounds
From MEPIS Documentation Wiki
If you find that you no longer have the KDE start-up sound and that none of the notification sounds are working then you might have been hit with a known bug in KDE.
In your home folder is a file called:
It should look like:
But something in KDE's system can turn these to false and that's the end of the game. Set it back to the above and restart the session (via a logout, end session) and all is well.