They took that away in version 15.6 of the app. Check out the release notes on why.
Dear Sonos,
can you please make easier the volume adjustment in the app?
Ore by the two buttons from my IPhone ore in the app with two buttons + ore - , you did have this after the last update but one button is disappear.
Thanks for your help, Jhon.
The volume adjustment did change this week. They removed the speaker icons at either end of the volume line which were useful to tap the volume up and down. Now there is only the speaker icon on the left, to mute.
However, as someone pointed out to me, you can still tap the volume line left and right to turn the volume up and down by one increment at a time.