OptionalcriticalThe critical alert flag. Set to true to enable the critical alert.
The name of a sound file in the app's main bundle or in the Library/Sounds
folder of the app's container directory. Specify the string "default" to play
the system sound.
OptionalvolumeThe volume for the critical alert's sound. Must be a value between 0.0 (silent) and 1.0 (full volume).
Represents a critical sound configuration that can be included in the
apsdictionary of an APNs payload.