It’s fascinating how the sounds of a city can be a part of defining its character, it can be a charm or a challenge, a boon or a burden. Earlier this year we were in Cairo, where the constant ear battering of traffic noise and raised voices soon becomes tiresome. Mexico City, in contrast, though just as loud and just as constant, has a soundtrack which is for the most part one of carnival and music. Chatter and laughter fills the streets, music drifts upward from every corner. It’s the sound of fiesta rather than frustration, joy rather than jams, an immediately exciting and enticing environment where the wall of…