Sound is air being moved - as demonstrated in a rather impressive way by Addi Somekh's balloon bass. Using a balloon and a string, he creates interesting sound reminiscent of a rubber double bass.
This simple construction principle is taken to the extreme by his Rubber Tree Quartet, where everybody plays an instrument made of balloons. More information about him and his instruments can be found on his website Balloonbass.com. There, he also offers a free download of his album Air, Rubber and Soul. Obviously he played his balloon bass for this album, but also invited a whole bunch of other musicians.
Here's a video of the balloon bass live on stage: