My suggestion is that "Pause All" actually pauses everything straight away rather than popping up a dialog box which asks "Are you sure you want to pause the music in all rooms?". Yes, I'm absolutely sure I want to pause all and when I do I'm usually in a hurry and I don't want to move my eyes and finger to the dialog. It not like "Pause All" is anywhere near the normal pause button on any of the controller apps.
Using a dialog box here is so early 90s UI/UX. Give users a little more credit. It not like it's a Delete All (you music) function which of course would need an 'Are you sure..?".
At least make the display of the "Are you sure you want to pause the music in all rooms?" dialog a configuration option in case you're worried someone will actually miss it and drive everyone else in the household mad with all their accidental "Pause All"s.
And while you at it, please consider adding 'Pause All' to the iPhone app's force touch shortcut menu.
The really big ask/suggestion would be to turn "Pause All" into a toggle which show "Unpause All" after a "Pause All" and allows you to quickly restart everything that was playing a few seconds ago. That would be better than the "Are you sure...?". I know that's 10x the work remembering the previous state (and forgetting it if someone else starts play independently). I don't really need this functionality but please stop asking me if I'm sure I want to pause all - I am.