Cheapest Chinese Bluetooth headphones from AliEx around 2019. I think it is an AC6905 chip. The toolchain is rather obscure and piecemeal with write-ups on GitHub and eevblog, but that is beyond my interest. This one was too weak to be useful or one of the audio drivers went out or wire was bad. I have a half dozen of these sports ear buds style headphones that have gone bad over the last decade. Even ones that come from the same brand end up having different boards.