Not really certain I understand what you’re getting at, but AirPlay 2 volume is an Apple thing, and completely separate from Sonos’ software settings. 

To clarify with an example, if I Airplay from an iPhone to Port playing content from the Apple Music app, will the Apple Music app volume still work even if the Port is set to fixed output? Thanks!

Again, the volume set on an Apple product is completely separate from the fixed volume setting on a Sonos. 

The Apple AirPlay 2 volume will continue to work, it is external to the Sonos. 

Well, my new Port arrived today. I can confirm that the Airplay volume set from the Apple device does not have any effect on volume output of the Port.  I tested Apple Music Airplayed from an Apple TV and an iPhone to the Port set at fixed volume.  The volume indicator on the TV and iPhone display changing volume levels, but no change in volume is audible from the Port. 

If you set the volume of a Sonos Port to be fixed, you will not be able to control the volume using the volume controls on your phone when using AirPlay. Instead, you will need to use the volume controls on the Sonos Port itself to adjust the volume. It's worth noting that the Sonos Port does have an audio line-in, so you could potentially connect an external device (such as a phone or tablet) to the Port and control the volume using the volume controls on that device. However, this would not allow you to use AirPlay to stream audio to the Port.