https://learn.microsoft.com/en-us/dotnet/api/system.speech.synthesis.speechsynthesizer.speakcompleted?view=netframework-4.8
SpeechSynthesizer.SpeakCompleted Event (System.Speech.Synthesis) | Microsoft Learn
Raised when the SpeechSynthesizer completes the speaking of a prompt.
event systemspeech synthesismicrosoftlearn