In its annual Ideas issue, the NYT Magazine notes that hybrid and electric cars are completely silent, which makes them dangerous to pedestrians, who can't hear them coming.
I certainly depend, almost unconsciously, on the sound of approaching cars while I'm bicycling.
In fact, I think people underestimate how noisy cars really are, being so used to them. I wouldn't mind things getting quieter, without sacrificing those unconscious cues we use to keep ourselves safe. The article wonders what sound would be best, but doesn't mention one that might be fun: the clipclop of hooves and the neighing of horses. With a little effort, we could have the soundscape of a 19th century street, without the smell- and turdscape.