Music has a long history as a weapon of war - it's earliest recorded use being the wall-tumbling 1400BC blast of trumpets used by Joshua in his seige of Jericho. In recent times, though, it has really come into its own thanks to US military and paramilitary forces who routinely employ amped-up musical assaults to psychologically destabilise an enemy. In Panama in the 1980's, for instance, the army attempted to blast Noriega out of the Vatican embassy with hard rock, and during the Waco siege, the FBI supposedly tried to unnerve the Branch Davidians with Christmas songs, clocks, and Tibetan chants. More recently, the musical weapon of choice in Iraq has been Metallica. To quote a US psy-op, "These people haven't heard heavy metal before. They can't take it"... To top it off, they even threw in crying babies...
And, as with any American battlefield weapon, there have been recent technological advances... Enter directed sound weapons, which can produce bursts of ear/head-fucking noise and music which will only be heard by people that they are targetted at. These weapons are already being used in Iraq and now, according to Boing Boing, they're being employed as crowd control in the streets outside the Republican National Convention in New York.
To get a sample of these devices in action, check out the website of their manufacturer, American Technology Corporation. On it is a primitive 3D animation demo and a San Diego TV profile. If you want more info about sound as a weapon, peruse the links on the website of Sydney sound artist, Alex Davies.