I actually have all I need to recreate the Sound Converter. The software was released under the GNU license.
// OFST is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
I have actually modified it somewhat to fix some issues with the ditch light capability, they didn't work right in all circumstances. I used a couple of these to add ditch lights to diesel upgrades.
The board layout and schematic is also available, but the board would have to be recreated in a PCB layout package, not a huge deal.
I still have a couple of the Sound Converters that I'm saving for a rainy day. I also wanted to be able to tinker with the software and needed one to experiment with.