First, credit due to Adrien (Yoda McFly) for the explanation of how this is done (it was done a few years back to an Arrows). If I mess the explanation up, it's my fault however
Simply put, every rev has a certain harmonic. You map each harmonic to the closest musical key (C flat, F sharp, etc.). Then you hook the engine up to a computer that 'plays' the engine by revving it to the appropriate key for the appropriate length of time.
The funny part of this is that a French car is singing 'God Save the Queen' (or whatever you Brits call that song...).
























