volume slider only affects master device

[cudaboy71]

M1 max studio mac running OS 12.3.1

i have two 27" apple studio displays (each with built in speakers); I wanted to create a multi-output device so that sound comes out of both.

while this app does provide volume control when the native macos sound panel doesn't, this app only seems to control the master device selected in the top pulldown of the audio midi setup panel.

is there anything else to be configured in the app or os?