Tinla put together a Ruben’s Tube sound-to-flame visualizer on the cheap netting some nice results.
The basic idea is that you have a length of pipe with hole drilled along the top. The ends are sealed, at one end there is a speaker and at the other end there is a gas source. Once the pipe is flooded with gas you light the vapour flowing out of the hole, crank some tunes into the speaker and stand back. The speaker creates waves of sound pressure through the length of the tube, and these variations in pressure cause the flow of gas through each hole to vary… resulting in a visualisation of the sound wave painted in flames.
Of course, pinching pennies while working with pyrotechnics is generally a very bad idea – medical bills cost a heckuva lot more than proper equipment! Ahem â€¦ that being said, check out the project page for more details.