If you are into gizmos, SoundRacer will sell you a kit comprising an amplifier and waterproof speakers which are linked via their software to the engine revs. You can choose from a range of high performance engine sounds to make your car sound like a Ferrari or whatever. Well that seems to be the theory but I have not to my knowledge heard one yet. The kit was developed for electric vehicles but they say they work equally well with petrol and diesel cars.
These were some prices provided a few months ago:
Prices in single quantity:
Connector and wire set for EVSMA-1
Connector and terminal set for EVSMA-1
Hall effect sensor with 4 magnets
EVSP31 Speaker square, 3", 25W
EVSP30 Speaker round, 3", 25W
They also do a much cheaper smaller version for use inside the car only, which seems a little pointless to me.