public void Sing(string vMessage)
{
    BackgroundWorker bw = new BackgroundWorker();
    bw.DoWork += new DoWorkEventHandler(bw_DoWork);
    bw.RunWorkerAsync(vMessage);
}
 
private static void bw_DoWork(object sender, DoWorkEventArgs e)
{
    SpVoice voice = new SpVoice();
    voice.Speak(e.Argument.ToString(), SpeechVoiceSpeakFlags.SVSFDefault); 
}
 
Sing("Hello");