It might be possible to plot the location of a single point in 2D, but how would you differentiate for multitouch? If you have 2 microphones you might be able to calculate the distance from each microphone of a touch by timing how long it takes for the sound of the tap to reach each microphone. However, as the speed of sound in wood appears to be ~4000 m/s (http://www.engineeringtoolbox.com/sound-speed-solids-d_713.h...) you'd need to be "listening" at 400kHz to get centimetre accuracy over a 1m square area.
I can't see how it would be possible to then differentiate between two points moving at the same time (i.e. a pinch gesture). I'm just assuming regular microphones are used as the article suggests - I'm not an acoustic engineer so hopefully I've missed something.