I am using the Debian testing branch.
After some upgrades system sounds suddenly disappeared.
How restore them?

Which Phonon backends do you have installed, and is Pulseaudio setup on your system?
Also, have you verified that all necessary outputs are unmuted and have appropriate volume levels set?

I apologize for delay in answering.
I really find it annoying to subscribe to topics that I create, most forums do this automatically.
Moreover, this setting is very hard to find.
So this is also about a feature request.
Anyway, what I have for notifications is "Built-in Audio Analog Stereo", and the backend is Phonon VLC.
Yes, pulseaudio is installed and I believe that it is set to per session, or something similar.
Thank you.

As far as I am aware, our settings are set to subscribe you automatically to topics you make posts to. Your settings on this can be set at ucp.php?i=prefs&mode=post

If you play a test sound using the Phonon control panel, does that result in any sound?

I changed the posting settings, following the link you posted.
The default was not set to Yes, maybe that is the problem, that it should be set to Yes by default.
Now to sound issues, if I click Test on the image below, the notification plays, so I don't know where the setting is to enable this on startup, shutdown, etc.
Thank you.
http://tinypic.com/1r547oyb

ckosloff wrote:The default was not set to Yes, maybe that is the problem, that it should be set to Yes by default.

It was set to yes by default for me.

Now to sound issues, if I click Test on the image below, the notification plays, so I don't know where the setting is to enable this on startup, shutdown, etc.

You mean the login and logout sounds? Those are disabled by default since KDE 4.11.
You can configure them and all other notifications in Systemsettings->Application and System Notification->Manage Notification.
Those two are to be found in the "Event Source" KDE Desktop (or is it "KDE Workspace"? I'm running in german here).

The source name appears to be "KDE Workspace" on my system for the logout and login sounds.

My sounds are back!