Well, I for one hope that no one takes this idea seriously. It’s not that hybrids in electric mode, or pure electrics, don’t make any sound. They most certainly do. It’s just that people aren’t used to the sounds they do make. Yes, they are quieter, but tires on the road and a certain amount of wind noise is inevitable. Once enough of these cars are on the streets people will come to associate the sounds they do make with automobiles. Right now, our brains tend to filter out the non-dangerous sounds and since these don’t sound like a car with a running engine our brains just don’t alert us. If we get to a point where electric vehicles are more common we’ll start hearing them just fine.
The idea of noisemakers on quiet cars reminds me of the old turn of the (20th) century laws requiring a man with a red flag to walk ten paces ahead and another ten paces behind any automobile. They were dangerous and might spook the horses you know 