I was told that it was a board problem because when they used a known working 4 slot mobo, the left speaker worked. I assumed this was true to save some time, so I unhooked all the harnesses and took apart the MVS. The bottom board of the pair is the one with the sound hardware in the area behind the volume pots. I cleaned that area but all the solder points and pins looked good, caps too.
Got out the MVS pinout and looked for which pin was Speaker L +. found that one, and then noticed that the pin on the edge connector had a trace cut with an exacto knife, and then the pin was jumpered to ground. I broke the jumper between GND and Speaker+ in the middle, and left the part of the wire connected to Speaker +. Then I tinned the other end of that wire, and connected it to the other side of the cut trace to restore it to new (I needed a bit of flux). Put the board sandwiches back together, inserted cartridges, and good as new!
