Reads a .wav file.
        Takes the path, and returns (PCM audio data, sample rate).

def _read_wave(self, path):
        """Reads a .wav file.
        Takes the path, and returns (PCM audio data, sample rate).
        """
        with contextlib.closing(wave.open(path, 'rb')) as wf:
            num_channels = wf.getnchannels()
            assert num_channels == 1
            sample_width = wf.getsampwidth()
            assert sample_width == 2
            sample_rate = wf.getframerate()
            assert sample_rate in (8000, 16000, 32000)
            pcm_data = wf.readframes(wf.getnframes())
            return pcm_data, sample_rate