Use the CancelAll
method.
private SpeechSynthesizer synth = new SpeechSynthesizer();
private async void Button_Click(object sender, RoutedEventArgs e)
{
await synth.SpeakTextAsync("You have an incoming notification from Big Bank.");
}
private void Button_Click_1(object sender, RoutedEventArgs e)
{
synth.CancelAll();
}