I hate this idea. Already enough noise pollution. How about human / obstacle detection, perhaps with automatic noise making if a collision may happen along with automatic slowing down, etc.. And in very high pedestrian traffic areas, slower driving speeds allowed.. etc.. People will learn new behaviour once their brain can't depend on the sound they make. This is one of those government things that shouldn't be legislated.
Just meet the requirements by playing the sound of clomping horse hooves.
This would be amazing.
This would be acceptable.
I grew up in a "small town", so I'm a bit paranoid, always looking around me when crossing streets, in parking lots, etc. I've found that people tend to walk around oblivious to the several ton devices propelled towards them. That's the world we live in. As we're a society that feels like we have to protect ourselves from ourselves (and litigation), I can't see a situation where we wouldn't have such warnings.