This MeFi post about number stations brought back some wonderful, creepy memories. I recall using my shortwave radio in my youth to comb the world for interesting stations. Often, I'd run across a "number station" - a station that consisted of nothing but a little music interspersed with a voice (often female) reading a series of digits, or an apparently random sequence of words. When I read William Poundstone's book Big Secrets, and confirmed what these were - transmissions used by spy agencies to send orders to field agents - I was even more freaked. And intrigued.
Having just finished Stephen Kinzer's Overthrow, which documents the history of American imperialism in the past century, I'm even more intrigued to revisit this subject. It'd be interesting (to say the least) to examine the ways that sound has been used both overtly and covertly in espionage and overthrow. Overt communication (propaganda) was a key tool utilized by the CIA with its Voice of Liberation broadcast in its war against Guatemalan President Jacobo Arbenz Guzman.