In some cases, the player would play sounds, but not Airplay.
Unfortunately, Airplay is using the Native Player of the tvOS device and there is not a lot the Bitmovin Player can do to control the behaviour.
To solve this, setting AUTOSELECT=YES on the audio track should resolve the problem.
If you are encoding your stream with the Bitmovin encoder, this can be set in the Audio media info configuration.
https://developer.bitmovin.com/encoding/reference/postencodingmanifestshlsmediaaudiobymanifestid
(autoselect = true)