Estou usando o SpeechSynthesizer da Microsoft para criar saída de áudio para meu aplicativo do Windows Phone. Eu uso esse código

private async void TextToSpeech_Tap(object sender, System.Windows.Input.GestureEventArgs e)
{
    SpeechSynthesizer synth = new SpeechSynthesizer();

    await synth.SpeakTextAsync("You have a meeting with Peter in 15 minutes.");
}

Eu recebo esta exceção:

{System.UnauthorizedAccessException: Access is denied.

at Windows.Phone.Speech.Synthesis.SpeechSynthesizer..ctor()
at NerdQuiz.Question.<TextToSpeech_Tap>d__1.MoveNext()
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.CompilerServices.AsyncMethodBuilderCore.<ThrowAsync>b__0(Object state)}

Eu descobri que o erro ocorre em

new SpeechSynthesizer();