I always just made my own, by making up an extension cable, standard NATO plug on one end, NATO jack on the other, and a stereo jack soldered to the speaker pins, using a small short cable coming out the back with the main extension cable. In short, I just wired in a small cable in parallel, with the left and right wires together, giving mono and not stereo, since the output is mono. Commercial devices for this will work, but are much more expensive. If you have absolutely no soldering skills, then you may have to pay for the commercial device.