In Microsoft TTS 5.1 or newer. The SpVoice.AudioOutputStream property says:
The AudioOutputStream property gets and sets the current audio stream object used by the voice. Setting the voice's AudioOutputStream property may cause its audio output format to be automatically changed to match the text-to-speech (TTS) engine's preferred audio output format. If the voice's AllowAudioOutputFormatChangesOnNextSet property is True, the format change takes place; if False, the format remains unchanged. In order to set the AudioOutputStream property of a voice to a specific format, its AllowOutputFormatChangesOnNextSet should be False.
It means a engine's always has a preferred audio output format. So, how can i get it.. i have not found any interface to get that attribute.