With the runtime of the speaker far exceeding my expectations I decided to see if adding a Bluetooth receiver would be viable.

Because the USB-C charging port on the speaker is also wired to receive analogue signal input we can use the port to both power a Bluetooth module as well as receive the signal.

Using a spare USB-C cable as a donor for a plug and a Bluetooth 5.0 MH-M18 module we can create a simple dongle.

The stereo output of the Bluetooth module is made mono by using two 150ohm resistors, the output is then soldered onto the DP1/2 and DN1/2 pins of the USB-C plug, VCC and GND have dedicated pads on most connectors. 

The board is then simply placed in a simple 3D printed cover.