I ended up having to use their Voicemeter product.
https://www.vb-audio.com/Voicemeeter/index.htm
Strangely my centre channel wasn't working so I had to hunt for Realtek drivers. Took some work but finally found a recent enough one that made the centre channel active. Still not working with Voicemeter in the preferred, WDM mode, but that could be because it is a pretty old chipset. Oh well, sound still seems in sync.
That took way to much work.