Walking for hours through the gritty streets in the center of Mexico City, you can hear the daily urban soundtrack: Car engines, the call of the man who buys scrap metal and the handbells that announce the passing of a garbage truck.

Walking for hours through the gritty streets in the center of Mexico City, you can hear the daily urban soundtrack: Car engines, the call of the man who buys scrap metal and the handbells that announce the passing of a garbage truck.