Processes a audio by adding it to accumulated state.

def _ProcessAudio(self, tag, wall_time, step, audio):
    """Processes a audio by adding it to accumulated state."""
    event = AudioEvent(wall_time=wall_time,
                       step=step,
                       encoded_audio_string=audio.encoded_audio_string,
                       content_type=audio.content_type,
                       sample_rate=audio.sample_rate,
                       length_frames=audio.length_frames)
    self.audios.AddItem(tag, event)