The system's notification sounds do not play either through use of the play button in 'Notifications' configuration module or when a dialog box/notification popup is generated by a program like Dolphin or Konversation if any video game is played.

Steps to reproduce:
1. Open 'Notifications' configuration module in 'System Settings';
2. Click on the play button for any notification type with 'Play a sound" checked to verify whether the selected sound is played;
3. Run Steam client and run any game from your Steam library;
4a. Go back to the configuration module and click on the play button again.
4b. Open 'Dolphin' file manager and select any item/file and then press 'Del' button to trigger a warning dialog box.
4c. Enable popup notifications for new messages in a channel in 'Konversation' IRC client and wait for someone to send a message in a channel, which for notifications are enabled. 

Actual Results:  
No sound was played

Expected Results:  
A notification sound should be played

To get notification sounds working back, just close the game and the configuration module or the program (Dolphin or Konversation as in this example) and then follow all the steps above except for 3rd one.