When two or more rooms are grouped together, dragging the volume slider left or right will automatically adjust the volume of all speakers in that group.
If you would like to adjust the volume on a particular speaker in the group, tap the volume slider once to view the volume of each individual speaker. Find the name of the speaker you’d like to adjust the volume for and drag the slider left or right to adjust its volume.
When a speaker is part of a group, tapping the physical volume buttons on the speaker will only adjust the volume of that particular speaker, not the entire group.