Since a couple of days, my system has taken to no longer playing notification sounds. And I don't know what I changed to make that happen. (My gut feeling says: "but I didn't change anything!", but you know, I probably did.)
The sound system still works. Music players, video players, web browsers, everything makes noises, except for the notifications.
In System Settings -> Application and System Notifications -> Applications, the sounds are still there. For instance, KDE Workspace -> Login shows KDE-Sys-Log-In-Short.ogg for a sound file; the "Play a sound" checkbox is checked, but the Play preview button does nothing.
Opening the ogg file in a music player makes it play, so the ogg file itself isn't corrupted. Unchecking and rechecking "Play a sound" doesn't help; neither does changing the sound file to another file.
Under the Player Settings tab, nothing has changed. "Use the KDE sound system" is still checked, the volume is still at 100%. Changing the selection or the volume and then changing them back (with an Apply in between) doesn't help.
In the alsamixer application, nothing is muted, so that can't be it either.
And I don't know what else to check. Can anybody provide more ideas to try?