Imagine a weapon that creates sound that only you can hear. Science fiction? No, this is one area that has a very solid basis in reality. The Air Force has experimented with microwaves that create sounds in people's head (which they've called a possible psychological warfare tool), and American Technologies can "beam" sounds to specific targets with their patented HyperSound (and yes, I've heard/seen them demonstrate the speakers, and they are shockingly effective).

Now the Defense Advanced Research Projects Agency is jumping on the bandwagon with their new "Sonic Projector" program:

The goal of the Sonic Projector program is to provide Special Forces with a method of surreptitious audio communication at distances over 1 km. Sonic Projector technology is based on the non-linear interaction of sound in air translating an ultrasonic signal into audible sound. The Sonic Projector will be designed to be a man-deployable system, using high power acoustic transducer technology and signal processing algorithms which result in no, or unintelligible, sound everywhere but at the intended target. The Sonic Projector system could be used to conceal communications for special operations forces and hostage rescue missions, and to disrupt enemy activities.

Here's the question of the day: if the military were to beam voices into somebody's head, what would they say?