def _list_audio_files(self, sub_dir=""):
        """
        Parameters
        ----------
        sub_dir : one of `needed_directories`, optional
            Default is "", which means it'll look through all of subdirs.

        Returns
        -------
        audio_files : [str]
            A list whose elements are basenames of the present audiofiles whose
            formats are `wav`
        """
        audio_files = list()
        for possibly_audio_file in os.listdir("{}/{}".format(self.src_dir,
                                                             sub_dir)):
            file_format = ''.join(possibly_audio_file.split('.')[-1])
            if file_format.lower() == "wav":
                audio_files.append(possibly_audio_file)
        return audio_files