MissingNO123
If a state with an AnimatorPlayAudio behaviour has a missing reference (i.e. the source audio file has been deleted from the disk) the build panel will show nothing under "Validations" and both build options will be greyed out. The only hint that something has gone wrong are checking errors in the console:
MissingReferenceException: The object of type 'AudioClip' has been destroyed but you are still trying to access it.
and
MissingReferenceException: The variable Clips of VRCAnimatorPlayAudio doesn't exist anymore. You probably need to reassign the Clips variable of the 'VRCAnimatorPlayAudio' script in the inspector.