Going back to the tree problem, when the tree fall it creates vibrations which travel in the air to our ears. The vibrations are channeled through the inner ear to the eardrum, which communicates the information to the brain as sounds. The vibrations create sound by making the eardrum physically move. Even if we are not in the wood, those vibrations are still present, and when they come into contact with other objects, cause them to vibrate and move on a micro scale, So small that the human eye could not identify the movement.
Davis et al, using a high-speed camera has managed to -at thousands of frames per second, pick up movement at the scale of a micrometer ( a 100,000th of a centimeter) from objects which have been exposed to sound. Even though the movements are much smaller than that of which can be represented by a single pixel, Davis' intuitive software looks for minute changes which occur in the whole image, which then translates these movements back into sound. In his TED talk, he demonstrates this by playing Mary had a little lamb through a loudspeaker to a plant and then plays the recovered sound from the imagery. It is far from the crystal clarity that we expect from modern gadgetry, however it is clear enough to hear the original tune, without straining.
Although in its infant stage, Davis has demonstrated its effectiveness using cameras at range and through obstacles such as glass and has even adapted the technology to run on a shop bought camera. Albeit the shop bought camera produces a less refined sound, it is still possible to identify the tune being played. However, through refinement of this method and perhaps enhancement of clarity of old silent film, we may be able to hear the voices of those who were once lost, or perhaps recover sound from a crime scene using CCTV footage or to be used as surveillance devices.
So in answer to the original riddle, a tree falling in the woods does make a sound when no-one is there to hear it (as we knew it would do). Maybe the question should be changed to "If a tree falls in the woods and no-one is there to hear it, or to have software analyse the video footage of the event, does it make a sound?"
To view the full TED talk, visit: http://cdn.instructables.com/FWH/W1CQ/G7DYJJSW/FWHW1CQG7DYJJSW.LARGE.jpg