Any pranksters out there? There might be enough time for you to design and build one of these musically unuseful ideas before April 1
st (and, yes, I did at one time fully intend to unleash both of these upon the unsuspecting world

):
1. The Swear Jar: an envelope detector triggers the replacement of the normal audio with a 1kHz "bleep" (optionally a different, more musical frequency). Bonus points for a remotely-activated design, especially one that could work with a microphone.
2. The B♭us (B flatus), or more discretely named, Beef Lattice: Notch filters remove B♭ frequencies and a farty sound of corresponding magnitude is inserted.