There is an option “volume control” in the app VitOS Orbiter for Silent Angel Z1. What does it mean? Does it make sense to control the volume of a streamer?
Of course not. That is exactly the reason why they don't do it.
If you read the screen a bit more carefully, you will see it says Setting for Internal DAC
This controls the RCA and the wired headphone out (that is analog, has nothing to do with streaming).
Do you really want a headphone out without volume control?