If in a parallel universe the first mass adopted automobiles operated silently, I don't think we would or should have eschewed them or put constant noise-makers on them. I suspect we would have done the same thing we actually did do, even in our world of gasoline automobiles: put a user-activated noise-maker on them and expect and require operators to use that device when appropriate.
However, I would also suggest that it is a very strange, unearthly, or extrordinary phenomenon to have such a massive and fast-moving thing traversing in such silence.
For example, there are not elephants that travel so silently that they may accidentally trample rodents underfoot.
Alternatively, though, outerspace and underwater, things are much less certain regarding the danger of large masses moving quickly.
The car could have a bell and "on your left" speaker. ;-)
As a cyclist, I enjoy having good situational awareness, and electric cars can definitely be surprising when they pass you. Overall, though I enjoy the super quiet ride, I think we should require some warning noise (engine noise seems like a nice option) to alert other to the oncoming car. It makes it safer for the driver because people are less likely to do something unexpected, and safer for the other people because they have better situational awareness.
Perhaps when automated car drivers are the norm, we can move to silent cars; since they will be more vigilant than a person could ever be.