I live a block away from Interstate 405. The roar of traffic never stops. It's more of a rush than a roar. I'm told I should imagine that it sounds like a waterfall or the sea breaking against a long beach, but to me it just sounds like 405. It has both volume and direction, and it is a backdrop to more local sounds.
A bird chirping is a sound against traffic, not a sound in itself as it would be in the stillness of a forest. You hear both; figure and ground are equally present. In a quiet place the silence has a sound, too, if only as the rushing of blood in your ears, but it is equi-present.
You have to find sound here, you have to sieve it out. It isn't presented on a silver platter of stillness. It is less precious, more contingent, and always on the verge of being overwhelmed by the aural context.
This, perhaps, is the urban experience. You have to work at extracting meaning from the other human artifacts. So much is lost because it's drowned out. And listening becomes pattern matching; you hear what you looking to hear. Like spread spectrum radio, you can extract a signal from below the noise floor if you know what you're looking for. Finding the unexpected is hard - even harder than usual - when there's so much interference.