Unscrew two tiny Phillips screws holding down the PCB (on the mini-USB side).

On the opposite side of the USB-Port (microphone side), the PCB is tugged under two latches. Pull the PCB from the housing by lifting it on its USB side.

Turning over the PCB will reveal the speaker cable pair, penetrating the case near the center axis. Pulling out these wires a little further reveals the culprit: the red wire is destroyed by chafing. Obviously, these headphones were not sufficiently tested for vibration.