A Boolean value that indicates whether the synthesizer is speaking.
- iOS 7.0+
- macOS 10.14+
- Mac Catalyst 13.0+
- tvOS 9.0+
- watchOS 3.0+
YES if the synthesizer is speaking or has utterances enqueued to speak, even if it is currently paused. Returns
NO if the synthesizer has finished speaking all utterances in its queue or if it has not yet been given an utterance to speak.