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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _load_audio_file(self):
 """ Load audio in memory. :rtype: :class:`~aeneas.audiofile.AudioFile` """

        self._step_begin(u"load audio file")
        # NOTE file_format=None forces conversion to
        #      PCM16 mono WAVE with default sample rate
        audio_file = AudioFile(
            file_path=self.task.audio_file_path_absolute,
            file_format=None,
            rconf=self.rconf,
            logger=self.logger
        )
        audio_file.read_samples_from_file()
        self._step_end()
        return audio_file