ok this one is pretty cool. we all know that air travels in waves, but because they are waves we have a pretty hard time actually seeing said waves. the Rubens Tube is a way to easily visualize these waves... WITH SCIENCE.
what is happening is, when a note is played on the speaker, the sount travels through the tube in its usual wave pattern. the wave contracts in some parts and expands in others thus creating the alternating heights of the flames.
the wave compresses the propane in some parts of the tube while expands in others, where the gas is compressed we see a smaller flame, where it is expanding, we see a higher flame.
this beats the crap out of the music visualizer on iTunes

btw, you might want to turn down your volume for this one