I remember hearing of a spy technique to recover speech in a room from outside the building by recording vibrations in the window.
This video goes to great lengths to show how difficult it is to recover sound from the movement of an object displaced by the sound waves.
At 1:59 he describes the perfect type of object to recover sound from - something very dampened, and very light, like a potato chip bag. So it moves in response to sound, but stops moving right away afterward.
At the very end is a better optimized recording of speech that is nontheless very muffled.
It works a little better with laser light - there's an audio "spy" recording at the end of this video.
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