MVS-4 confusion!

So I have 2 MVS-4 slots but their layouts are slightly different. One has both volume sliders on the right side (when the jamma/mvs connector is facing forward) and the other has a headphone slider on the right, while the main volume slider is in the front, next to the jamma/mvs connector.
The 2nd board I described has 2 headphone jacks off the the right of the PCB, but they produce no sound. I was just curious, if there was a way to enable them. Looking at the chart on hard MVS, 4 slot MVS do not have headphone jacks, which is what is confusing me.
My main goal was to upgrade the speakers on my Neo Candy 29 (the speakers SUCK!!!!!) possibly with some good PC speakers and the headphone jacks would have greatly simplified this process. Any help here will do.
Let me know if you need anymore info,or pics.
Thanks!!!

There are at least two different versions of the 4 slot board.

The 3.5mm stereo plugs are not headphone jacks, these were intended to be used for linking up games but I don't believe anything has ever used them as all games I know of that link up have jacks on the actual cart.

PC speakers are crap. Use car speakers (at least 3-way).

Ah thanks for setting me straight!
Yea, PC speakers are crap. The space in the candy 25 is limited tho and I don't really know what car stereo speakers would fit nicely. 3 ways would sound AWESOME!

Cut out a piece of paper the same size as the original speaker and take it down to the shop.