Ok so I have an idea:
A musical instrument MADE OF ENGINES.
That is: un-muffled internal combustion engines, where the explosions make both the beats and the tones! Its an idea who's time has come, with the advent of Dubstep!
The most obvious (of doubtlessly many) problems: the tone an engine produces is determined by its RPM (as well as size), and an engine would have to rev through all RPM's between two tones in order to change the note. This would make for boring music, most likely. I can think of several solutions to address this:
1. Electronically controlled ignition/fuel injection. The engine doesn't have to fire on every stroke. This could be used to make percussive, not necessarily regular beats at low speed, or to increase or reduce the pitch by factors of two at higher speeds. I'm assuming v-twins would be used (obviously).
2. Spin the engines to the needed RPM's with electric motors. When they're not firing, their speed could be adjusted to whats needed to create the next tone.
3. Number two would work best if there are several engines available, so that they can alternate which one is firing while the others are adjusting their speeds to match the next note. This probably implies that it would only be good for pre-recorded music, though it might work with a delay live. Unless you just had enough engines being spun at different speeds that you could make any tone you wanted; like an engine organ. Different size engines would also naturally give a different range of pitches.
4. Since the energy of the combustion isn't being used to maintain the engine speed, the volume of each explosion could be varied by adjusting the air-fuel mixture. The electric motor might even need to brake the engine to keep it at the right speed.
Fuck yes. Now I need a bunch of engines, some electronic stuff, and more skill with programming than I have.
How many engines? Well, twelve, if I wanted to do the organ thing, where each is spinning at a speed appropriate for a tone. That would give all the semitones in one octave, which is probably overkill. Since you're likely to want to be playing in some key, eight is probably sufficient (or maybe even just seven). The trick with the electronic ignition would give you at least a three-octave range without changing engine speeds; and that can also be adjusted on the fly. Though, I suppose I don't know what range of speeds the engines could actually support without exploding. Actually I do know that, I just don't know what tones they would make.
Another thing: V-twins would require extra cooling, assuming (obviously) that I'd be using air-cooled engines. They'd have to be wrapped in copper tubing and have water running through it, which is also pretty cool.
Oh and finally: The exhausts will be brass trumpets. Actually if there were valves on the trumpets they might have some control over the tone as well...