Unfortunately, Assistant-Relay doesn’t really support all of the functionality true voice commands seem to support. This is an issue with the NodeJS server/Google SDK, not an issue with the Hubitat side of things.
It’s been a while, but as I recall commands in the following form sometimes work...
[CC]set volume to 50 on Family Room Mini
But I am pretty sure the volume command above does not work. It does if you use Google Assistant on your phone, though (without the [CC] of course).
I don’t recall ever having to use the wake phrase “OK Google” when using Assistant-Relay.