Playback volume with AirPlay is limited to the volume setting in the Volumio Web UI.
If the volume is set to 25% in the Volumio Web UI, music playback with AirPlay will not go louder than 25%, even if the volume is set to maximum on the AirPlay device.
Expected correct behaviour:
I think the correct behaviour should be that the volume setting of the AirPlay device is independent of the Web UI setting.
This bug is a problem if I listened using Volumio Web UI and left the volume at a very low setting (say 5% or so). My kids will then not be able to set the volume high enough for their purposes from their iPods via AirPlay. They would have to fuss around with a web browser to change the volume in the Web UI, which is not an option (for various reasons).
This issue has already been reported a while back, but it seems nothing has changed since then (see https://volumio.org/forum/#p25133).