[Certified]

Most speakers can operate as a microphone. You just operate them in reverse. Sound waves in a room move the speaker cone which in turn induces a voltage on the speaker coil as it vibrates. In most modern speakers the amplifier circuit gets in the way of making anything of it. "Passive" pc speakers that didn't have an amp used to be more common in the 90s. Usually a cheap set came with a new computer purchase. They didn't have the power/wall wart and ran solely off the power pumped out by your computers 3.5mm audio. My mind was blown when I found out as a young kid that you could plug these into the microphone port and record... from a speaker!

[IIAOPSW]

pfft. now the clever hack is using your camera as your microphone.

[squarefoot]

Not a camera, but a cellphone potentially yes. The MEMS accelerometer contained in all phones is a device that turns position and acceleration (therefore vibrations) into digital values, so they could be sensitive to sound, especially when put on surfaces that resonate easily to ambient sounds and voice, like a table. Most if not all accelerometers are however sampled at much lower frequency than sound, then have their signals fed through a low pass filter, which could be either or both digital and analog, so in theory it is possible, still very hard to achieve without some deep hacking. However, if a cellphone was manufactured to allow it, then any app aware of this would have the ability to listen to sounds without involving the internal mic.

[kimburgess]

Like this? http://people.csail.mit.edu/mrub/VisualMic/