STUMPED by Mini-Commander EX and sound

Should have researched the forum in detail before I started this project but here goes in case somebody has an idea:

Decided to put sound in a Williams E-8 dummy B unit I have and the obvious choice (without looking at previous threads here) seemed to be an ERR Mini-Commander EX and one of their new Railsounds Commander boards although I have one of their older RS4 boards too. I have used the EX before.

This is the diagram of the EX outputs:

I was bemused by the fact that (a) the Railsounds Commander manual does not include a diagram showing the connections between the EX and that sound board and (b) all the connections for other control boards that are shown include a serial return, which does not figure in the EX wiring. To be frank I don't know exactly what serial return means/does in this connection, and so long before actually starting this project I contacted technical support and was told that the EX connections are similar to the Mini-Commander 2, which are shown here (page 23 of the Railsounds Commander manual):

The difference appeared to me to be that the Mini-Commander features one wire connection to Railsounds although I probably should have realized that that absence of any serial return/chassis ground meant something.

Anyway, on wiring up the EX and Railsounds Commander as suggested and applying power to the track the sounds came on having defaulted to conventional control. No amount of TMCC sound commands produced any difference, nor did checking all the connections. I knew that there was a TMCC signal to the EX because it responded to "SoftSet" commands and the light I attached to the headlight output in order to test it responded to AUX2 on and off commands but the sound commands appeared not to be getting to the Railsounds Commander board.

I wasted a fair bit of time trying to diagnose this until, belatedly, I found a thread on this forum over a year old in which it was confirmed that "The Mini Commanders (ACC, ENG, EX) supply RS serial data, but note the output drive level is limited which works in most configurations, but not for the latest RS upgrades offered at ElectricRR.  We have signal boosters available to overcome the limitations of the drive levels . . ."

This was from Jon Z and other real experts like Boxcar Bill and Gunrunner John also weighed in.

So the configuration I have got appears not to work for the purpose of putting sound in my unpowered B unit and I can't find any other reference to the signal boosters mentioned. So now I am driven to ask: Does anyone know what combination of available boards would work? Is there any TMCC board (or boards available) that would drive the Railsounds Commander in an unpowered unit? Should I try running the EX with the older version of the ERR RS board I have or is that doomed to fail too?