The Trololololo Man add-on lets you set a time interval or to specify if you want to be taken by surprise, with a randomly played melody.
The Trololololo Man extension was inspired on the many videos of this guy on the Internet and based totally on the fine extension Consciousness Bell by Ronen Zilberman.
NOTE: The Trololololo Man add-on is cross-platform and it works on Windows, Linux and Mac OS.