The standard English reflex when speech is muffled or unclear is a sharp "What?". Dropping a blunt "¿Qué?" in Madrid, Bogotá, or Mexico City lands with abrasive finality. It signals annoyance, disbelief, or aggression rather than a polite invitation to repeat a sentence.
Conversational Spanish uses ¿cómo? for acoustic misses. If an interlocutor speaks too quietly on a subway platform, ¿cómo