In the capital of Madagascar, an island nation where nearly 80 percent of people eke out a living on less than $2 a day, an unexpected sound emerges from the slums of the capital, Antananarivo: Zaza Kanto, an a capella group of boys, ages 11 to 16, whose joyful sounds and viral videos have elevated them to national fame. They’ve also earned praise from Fredo Austin, of the band Freedom’s Boombox, who mentored the young men as part of an exchange program called American Music Abroad. Seth Doane reports.