Sometimes Latin American dance tunes on the radio — salsa, cumbia, ranchera — bring a little cheer into the emergency room of Mexico City’s Ajusco Medio hospital, which is operating well over normal capacity because of COVID-19. Dr. Marta Patricia Mancilla, head of the emergency unit, says the upbeat soundtrack is a distraction from the routine at the packed hospital, where some people have kneeled at the doors of the emergency room, praying for …