Okay I did make some progress and the soundplus does indeed work after some chip swapping, but the results are very strange so time for a sanity check because it has me scratching my head:
Here is what I do know:
2 sets of Xenon vocalizer/sound boards 1 working (mine), 1 not working (Timab's). From here on out Timab's will be referred to as H and mine as M. Vocalizer as V and Soundboard as S.
MV -> MS = working (baseline)
HV -> HS = non working
HV -> MS = working
MV -> HS = working (after swapping out known good U3, U10, U4 on the soundplus - Oddly U3 works in MS, but does not work in HS - you can hear the sounds in the test, but it is all just static and garbled. If I put it in mine, it works fine, and if I put mine in his, it works...)
I cannot get both of his boards to work together. Both cables do work so it definitely isn't the cable.
What I did find is that the soundplus C27 is actually good, but on both sets of boards to get any reading out of it MV must be attached. It does not give a reading with HV attached. (diode check - I know not the actual way to check a cap but it works when doing comparisons). I see the same outcome if I attach HV to MS. C27 does run to U4, and a path to pin 8/9 of J3 has an interaction.
Regardless though, I can't quite wrap my head around why I can get both his boards to work if attached to my boards, but not his boards together. I assume at this point that there is something else wrong on HV, but like I said, it works 100% with MS.
TL/DR: His boards together don't work. Either of his boards with either of my boards work 100%. I had sort of tested this before, but I hadn't swapped any chips in the soundcard at that point so it didn't work with either vocalizer the last time I tried.