Skype does not ring in Ubuntu

Skype does not ring... More precisely, all events that display notifications fail to play sounds, but the others work fine (e.g. testing the event of "Ringing out" plays the sound).

Solution: System -> Preferences -> Sound

Pull up the Alert volume like above.

Thanks goes to http://ubuntuforums.org/showpost.php?p=10084372&postcount=4