criticalSoundVolume property
The volume for playing the critical alert sound.
The value is between 0.0 and 1.0. This is only used when the notification is configured as a critical alert. Document: https://developer.apple.com/documentation/usernotifications/unnotificationsound/criticalsoundnamed(_:withaudiovolume:)
Subject to specific approval from Apple: https://developer.apple.com/contact/request/notifications-critical-alerts-entitlement/
On iOS, this property is only applicable to iOS 12.0 or newer. On macOS, this property is only applicable to macOS 10.14 or newer.
Implementation
final double? criticalSoundVolume;