For a long time I had a broken Bluetooth headphone model HP BH10 which I didn't want to throw away because it's electronic part is working just fine so I decided to put that electronic part to use and make the best out of it and the first idea that struck my mind is a speaker because headphones almost contain most of the components needed for a Bluetooth speaker.

Supplies

Broken Bluetooth headphones
An audio amplifier of your choice. I'm using a PAM8403 5V 2-Channel Stereo Audio Amplifier Module since it is easily available and cheap.
soldering iron
Micro usb type-b breakout board
Power bank

Step 1:

First remove the head band from the headphones

Step 2:

Now in one of the ear cups you will find the electronic part of the headphone and it may look like something in the above figure.

Step 3:

The only pins that we care about are L+,L-,R+,R- as they are the audio outputs from the Bluetooth and we need to amplify that signal using a amplifier with external power and connect them to the headphone speakers.