Represents a critical sound configuration that can be included in the
aps dictionary of an APNs payload.
The 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
folder of the app's container directory. Specify the string "default" to play
the system sound.
The volume for the critical alert's sound. Must be a value between 0.0 (silent) and 1.0 (full volume).