I would recommend the method Taito used on a lot of their JAMMA boards.I'm not an audio engineer so I'm not quite sure yet the best way to adapt the sound to jamma.
the Sound wiring is really clever it uses the Audio negative from the JAMMA for the positive on the right speakers, and then the audio positive from the JAMMA as the negative on the left speakers. with the reamaining terminal on each speaker wired to DC ground. At first I thought they were being wired out of phase but because they're using the audio negative as a separate channel they need to wire those speakers in reverse to bring it back into phase.
so basically
Left Channel + to speaker +
Right Channel - to Speaker -
so long as the cab or super gun isn't grounding out speaker - this will allow mono cabs to play the output in mono, and Taito Cabs (like Egret II and Egret 29) to support Stereo through the JAMMA connector.