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 …
Interestingly this happened to us this past weekend. Got really low volume out of our Air. Eventually solved it by, yep, turning up the analog volume dial on the front. Somewhere one of the kids must have played with it and turned it down because i never had any issues in the past.
Only have this problem/feature when using the analog cable which is rare (only when i hook up Sonos to it). Otherwise, i use bluetooth to connect and the volume dial is moot/non functional.