For those stun like me, couldn't figure out how to group Air's together for multi-room sync here is a quick video. Drag and drop. Once you do that you will see a new master volume. Personally I'd like to see the Big main volume slider on top of the page be the master.

https://youtu.be/dkzJ8mldtIc

Sync and play one usb thumb drive or nas drive etc to entire setup.

2xAA
M80/VP150/VP160/QS8
4xM3/M22OW/M2OW/VP150OW/4xM3IC/4xM3OW
EP500/800/HSU VTF3/SVS PB2k/SB2k/SB-12