As with restrictions on neon lights on lowriders and things like that, something that would be too loud or distracting to other drivers would probably have rules against it, although I hope that the restrictions aren't too onerous. I'd like to see some creativity. Classical music?

A lot of thought goes into startup chimes and things like that for computers. If people worked on this problem for a while, I imagine a sort of "basic" sound would develop, like a "basic" headlamp, a pleasant, calm hum that changed frequency slightly with acceleration and deceleration.

If you worked it right, a slow line of traffic would sound like a lazy summer afternoon in a clover field among thousands of bees...