One common Bluetooth module is the CSR8645, which [lorf] realized was fairly common and (more importantly) fairly easy to modify. [lorf] was able to put together a toolkit to reprogram this ...
The headphones pair a CSR8645 Bluetooth audio receiver with a TP4056 USB-C charging module, a 500 mAh LiPo pouch battery, a pair of Dayton Audio CE38MB-32 drivers, and replacement ear covers ...