PHP:I love the idea of sound as a weapon -
I remember reading somewhere once that the British army once tested infrasound or ultrabass or whatever on protesting crowds in Northern Ireland. Apprently because a certain level of it can induce nausea and sickness ie; pretty hard to protest when your throwing up. What a dark image.
the americans used sound weapons in new orleans to control the crowds
this one http://www.npr.org/templates/story/story.php?storyId=4857417
there are all types of non lethal weapons http://en.wikipedia.org/wiki/Sonic_weapon
One nasty one that gives people the feeling their skin is burning, this is apparrently for crowd control, which seems far more likely to cause mass chaos.