Hyundai Motor has developed a sound system for vehicles in which passengers and the driver can hear different songs or make totally private phone calls. It is the first such system in the world, the carmaker announced Sunday, and will be offered in Hyundai Motor cars within a year or two. Korea’s largest carmaker calls the system a “separated sound zone” and unveiled it on its official website and on YouTube last Sunday. In the YouTube video, classical music plays in the front of the car where two parents are sitting, while pop music plays in the back for children....