I may be putting words in Andy's mouth, but I think you are looking at things from the wrong end.
The JAMMA edge connector connects to the speaker output (and JAMMA is mono) and the screw connectors are for input, which needs to be amplified mono. One of the way people do that is to hack PC speakers to get at the amp circuitry (I use little $2.5 BIY amp kits)..
If are going to use actual PC speakers, you bypass the JPac altogether.