Hoof Beats? Really? At 15 MPH or less? If Mercedes-Benz can make sensors that allow a vehicle to stop short of a vehicle stopped or to maintain distance during cruising, why can't the same sensor array be used to flash headlights or the audible double honk warning prior to the vehicle stopping on it's own. For that matter, how about day/night vision that compares digitally comparative movement from outside of the planned vehicle path based upon steering position. Making noise to prevent accidents is like putting playing cards into the spokes of my bicycle, it might keep those with hearing out of the way, but what about grandma or the late night pub crawler?
I can certainly understand the need for this. I have an issue with hearing and EVs, depending upon the model, are almost "nonexistant" when unseen. In the work I do on a daily basis; i.e. gas combustion, its'a know fact that natural gas is oderless so, an oderant is added for detection. Very same principal as Charles has mentioned. The automakers are definitely following the logical path, even if my mandate.
Believe it or not, oldpartsnrust, there was a movie called "The Dilemma" with Vince Vaughn where the main characters do the very thing you've described. They develop a way to make an electric car sound exactly like an old-fashioned muscle car.