Why is my music still playing out loud with headphones in?

Ask China...na them get the perfect answer for that question.

Try your headphone on another phone to know where the problem is coming from. It's either your headphone has become faulty or your phone needs some settings done to get it working properly.

Does the sound come from both the phone and the headphone at the same time or just the phone? If it's the former, then it's very likely that the headphone has become faulty, precisely the jack of the wire.