Master volume controls don't work when using AirPlay

When an AirPlay device other than the Mac's internal speakers is selected as the audio output, the main/master volume controls in Finer Music Player no longer have any effect.

The only way to adjust the volume of the AirPlay device is to open the AirPlay menu and adjust it there.

Expected behaviour:
The master volume controls should continue to control the volume regardless of which output device is currently selected, including AirPlay devices.

This would also be consistent with the behaviour in iTunes, where the main volume control can be used to adjust the volume of the active AirPlay output.