I’ve not seen the issue @controlav mentions, but note the two different screens/forms shown in my earlier attachments. I’ll just refer to those screens as:

• The Slide-Up Room/Group Selector Screen
• The Output Selector/Room Grouping Screen

In the latter screen, the volume slider control only shows when a room is selected (checked). See the attached example taken from my iPad controller App. Hope that assists too.

Sometimes the volume slider will be missing from a speaker in the list, which makes changing its volume challenging.

And perhaps see the attached from my own Sonos App…

Maybe see if the below might assist…

Sonos App User Guide:

https://www.sonos.com/en-us/guides/sonosapp