>> Maybe someone could find a way to convert the sound energy from an engine
>> into mechanical or electrical energy...
>
> Some do, in a way: Resonance exhausts. They're designed to use the 
> acoustic
> "bang" produced when a cylinder is opened to expel the exhaust gases, to
> resonate in the exhaust system in such a way that a lower-than-average 
> pressure
> is achieved when the next cylinder opens, helping expel it.
>
> Probably must be tuned to some certain rpm optimum though.

Oh yes, that is done already in most (all?) modern cars, the length and 
geometry of how the individual pipes from each cylinder join up together is 
very important in making an efficient and powerful engine.  In road cars of 
course efficiency is usually the design goal (and the designed rpm is much 
lower) rather than overall performance, but sometimes in sportier cars and 
racing cars you definitely really tune the exhaust for a high rpm (usually 
around maximum power revs), this results in the exhaust system getting 
noticeably louder at a certain point.

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