@AntEater and @timlance I came across this topic searching for how to output audio to AirPlay (or even anything but the internal speakers on my MacBookAir) and I finally figured it out and am posting my findings here for anyone else who might search and find this topic:
Preferences/
Backend/
Under "Audio Output":
Output: "Output to a sound card on MacOS"
Device: "Automatically select"
Of course, you can select any of the available devices under the "Device:" menu, but the "Automatically select" option appears to follow what output option (including AirPlay speakers) I have currently selected under my "Sound" menu options on the menu bar.
Note - any changes in preferences didn't go into effect until I stopped and started whatever I was listening to.
Great job developers!!