My iphone get warm while listening to apple music

Using an iPhone to stream music, whether using Wifi or Cellular makes the battery work harder. When a battery works hard, it generates heat, which is what is called thermo dynamic exchange. This is not concerning, nor an issue. The phone has very sophisticated circuitry built in to prevent it from overheating. Normal operation won't overheat the phone. If the phone actually overheats, an image of a thermometer will display, with a warning the phone can't be used until it cools down. Otherwise, accept that pushing a battery will generate heat.