check your headphones again. Take your multimeter and measure the charging port (circuit) while it’s plugged in, if it’s work properly, you will measure that voltage, what is mentioned on the adaptor (or in the tech. spec. of charging technique).
DO NOT TRY IF YOU ARE NOT SURE ABOUT YOUR SKILLS!
You can also check your battery if you have a laboratory power supply (generator), set the current to the maximum of the charging current (usually according to 1A up to 2A - SET LIMIT OF YOUR POWER SUPPLY CURRENT) and set the nominal voltage to the battery voltage INPUT (usually 5V up to 24V). Connect your battery pins to the power supply output, PLEASE BE CAREFUL! BE SURE, YOU ARE CONNECTING TO THE RIGHT POLARITY! and run your power supply. After some minutes (5-10), your battery would be charger enough. Insert your battery back to the headphones and try, if it’s charged. If not, or if it’s according to 1% of capacity, your battery is faulty. Also, while your battery will be connected to the power supply, I recommend you to periodically check the battery temperature and state, if the battery is overheating, do not continue in the charging! Your battery could explode! The same case, if your battery is about to inflate, in those cases, your battery needs to be exchanged.