If memory servers, there is an old William Gibson novel where a kid uses similar technology and uses sound to kill those around a girl he is stalking. When the hero of the story gets ahold of the device, the reader learns the sound was a recording of the girl's voice saying "I love you".

Also, imagine advertising uses.

Want to dispurse a crowd? How about 200 dec. of nails on a blackboard.

Because only those in the "line of fire" get their heads exploded, those on the sidelines feel no pain and can happily justify fellow citizens pain through whatever rationales the goverment provides.