Could the road surface be treated in such a way that rolling tyres will make enough noise to alert pedestrians? eg a less severe version of rumble strips. It should only be necessary in areas were pedestrians and traffic co-exist (where speed is low).
Finding the right sound is certainly a very complex task, since inside and outside sounds may well be different. Additionally, maybe having the driver conscious of some of the other sounds made by the vehicle might be a good thing, possibly making it easier to recognize when there is a potential problem starting.
Some visually impaired people are already complaining that hybrid cars in pure electric mode are so quiet that they don't hear them coming. Some external sound for vehicles would seem to provide a safety factor for pedestrians. It will require some regulatory oversight - if people can create and download "car run sounds" then misleading sounds like the sound of blowing wind, an ambulance, or running water could pose a threat to public safety. Certainly inside masking sounds would enable vehicle occupants to avoid being annoyed by random vehicle sounds - and probably require much less oversight.