Instructs the speech synthesizer on how to generate the
output audio content. If this audio config is supplied in a request, it
overrides all existing text-to-speech settings applied to the agent.
The synthesis sample rate (in hertz) for this audio. If not
provided, then the synthesizer will use the default sample
rate based on the audio encoding. If this is different from
the voice's natural sample rate, then the synthesizer will
honor this request by converting to the desired sample rate
(which might result in worse audio quality).
