You can here them everywhere you can walk up to a cliffside and look down.
But I too don´t know what it is that´s stomping around down there.
As I was on the Noob-Isle, someone said that it is the Sounds of the Guns, from the Main Continent, carrying over to the Noob Isle. *g*
Same applies to the Citysounds, those are in every City, but different in each.
Its simply a low "voice drone". It´s many, many diferent voices laiered over each other. The "ambient" sound of the Big City.
If you walk into a mall, what do you hear?
The murrmurr of a thousand voices, fused with all other sounds around there. The Lack of "easy listenig" music, escalators etc, in Yrkanis, Pyr, Zorai, or Fairhaven simply let´s us hear a mix of the Citisens voices. No more, no less.

I find it very lifelike, except for the repetition, every minute or so.
Acridiel