Taking the speaker that is working, trace the wires back to the B2.
Unplug the other pair of wires (to the speaker that is not working) and leave them out.
Then plug the working speaker wires into the other sockets and see if the speaker still works.
If it does, then the problem is with the wiring for the non working speakers.
If it does NOT then there is a fault with the B2.
Fred