Have you (or your PC, if set to install updates automatically) recently installed any Windows Updates?
(Assuming its a Microsoft operating system).
Sometimes, these updates tend to kill the 'system sounds' settings - on one occasion my sound card was disabled without me going anywhere near the control panel settings. I have frequently lost my Recycle and Incoming Mail Notifications without messing around either!
I suggest that you look in Control Panel - Sound, then open the Sounds tab and look through the assigned sounds under Program Events (Win 8 or 8.1) or similar in other versions. Reset the desired sound effect, test it using the 'Test' button then and save and exit.