Amazing show on Discovery channel about "Stealing Secrets", much of it related to audio espionage. One highlight; Mr. Theremin was a high level Soviet spy who was sent to USA by Lenin and Stalin to steal USA patent info while filing Theremin related documents at the Patent office!
Lenin was so amazed by his personal demo of the theremin that he learned to play music on it himself. Stalin sent Mr. Theremin to Siberia during the early purges, but later re-elevated Mr. Theremin to another high level intelligence position.
as a general comment, I'm amazed we "won" the cold war. The Ruskies developed audio espionage devices that still seem impossible 50 years later. (totally passive, only active when hit with a 300mHz signal, undetected for years). The best USA could do was multi-million dollar Rube Goldberg type devices that did everything but play the James Bond theme in the background.