Update the audio length property,
        according to the length of the current audio data
        and audio sample rate.

        This function fails silently if one of the two is ``None``.

def _update_length(self):
        """
        Update the audio length property,
        according to the length of the current audio data
        and audio sample rate.

        This function fails silently if one of the two is ``None``.
        """
        if (self.audio_sample_rate is not None) and (self.__samples is not None):
            # NOTE computing TimeValue (... / ...) yields wrong results,
            #      see issue #168
            #      self.audio_length = TimeValue(self.__samples_length / self.audio_sample_rate)
            self.audio_length = TimeValue(self.__samples_length) / TimeValue(self.audio_sample_rate)