def _list_audio_files(self, sub_dir="<STR_LIT>"):
audio_files = list()<EOL>for possibly_audio_file in os.listdir("<STR_LIT>".format(self.src_dir,<EOL>sub_dir)):<EOL><INDENT>file_format = '<STR_LIT>'.join(possibly_audio_file.split('<STR_LIT:.>')[-<NUM_LIT:1>])<EOL>if file_format.lower() == "<STR_LIT>":<EOL><INDENT>audio_files.append(possibly_audio_file)<EOL><DEDENT><DEDENT>return audio_files<EOL>
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`