Although I have not looked at this for a while … the air can provide its own Wi-Fi hotspot. It needs to be enabled in the app first … set gear->network->hotspot to ON. Then you can use the app to control the volume directly to the Air.

I’m not sure of the intent of volume knob on the front of the air. I just leave mine at max. So, iPhone volume at max, volume knob at max, app volume in control.
I didn’t actually test this … so you need to …