It sounds at first like a cry for help, the plaintive voice of a woman in distress. (Or is it a girl? And is she really in need of rescue? It’s so hard to tell at this distance.)

But then, as it gets closer and louder, the words become clear: “Se compran … colchones … tambores … refrigeradores … estufas … lavadoras … microondas … o algo de fierro viejo que vendan!”

It’s a brief recording, blasting over and over again from a loudspeaker on top of a truck rolling slowly through the streets of Mexico City. The people in the truck are recyclers, looking to buy used mattresses, home appliances and “old iron things.” And as they pass, that voice, piercing the air at maximum volume, eclipses the rest of Mexico City’s relentless soundscape and even your own thoughts as it loops and loops and eventually fades into the distance.

Amid the noise, many here wonder, sometimes in curiosity, though often in annoyance: Whose voice is it?

It belongs to María del Mar Terrón Martínez — or it did, when she was 10 years old. Her father, Marco Antonio Terrón Aguilar, is a scrap metal collector, and he used to solicit the stuff by yelling through a megaphone. But it got to be tiring, and it was chewing up his voice, he said in an interview, so one night in 2004 he asked his daughter to record the message for him.

That recording has since been copied many times, and is now used by hundreds of scrap metal merchants all over Mexico, in other Latin American cities and even, Mr. Terrón reports, in the United States. And it’s become a signature cry of Mexico’s capital, the crowded home to about 21 million people.

The streets of the city are alive with all kinds of vendors, many of them ambulatory, and all with their own tactics for cutting through the auditory thicket, from recordings to pan flutes to hand bells.

A friend who was raised in the backwaters of southern Virginia has been having a hard time adjusting to the sounds of the Mexican metropolis. “Cacophony,” he calls it.

I prefer to think of it as an elaborate choral symphony, with the street vendors as its soloists, providing the soundtrack for a restless city that’s going places.