An update on the situation:
I now verified that after setting the audioMode to Default or VideoRecording explicitly, I can play the audio using AirPlay on an Apple TV.
However in my project, I need to play it on a speaker with AirPlay functionality on Airport Express. Thus, there is no video stream.
When using the Default mode or the VideoRecording mode, the Airplay speakers don’t even show up for selection, however the AppleTV does. When set to other audioModes, the speakers show up (as well as the TV) but the audio does not stream to any of them.
I think we need to use voiceChat mode, but I don’t know why it’s not working properly.