Sounds are signatures, identfiers, a species of audio trademark. Like smells, they can lurk mysteriously in our consciousness evoking strong memories or dreams at odd and unsuspecting moments. They have a power to stir us far beyond the tidy outlines of sight.
No one is amazed that an outdoorsperson can identify birds by their calls or know the needs of a squirrel or coyote by the noises they make. Why should we think it weird that urban and suburban dwellers can discern their neighbors by the sound of their idling car or the whoosh of tires on pavement? The mail van, garbage truck, landscaper's trailer, and UPS vehicle all make sounds as distinctive as their missions. More often than we think, we know what's going on without looking up.