code: def _list_audio_files(self, sub_dir=""):
          
          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
docs: 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`