The following code uses the os.walk() method to recursively walk through all the files looking for  .wav and .mp3 files, as follows (see code 5.03—view.py):

def add_all_audio_files_from_directory(self):
  directory_path = tkinter.filedialog.askdirectory()
  if not directory_path: return
  audio_files_in_directory =  self.get_all_audio_file_from_directory(directory_path)
  for audio_file in audio_files_in_directory:
     self.model.add_to_play_list(audio_file)
     file_path, file_name = os.path.split(audio_file)
     self.list_box.insert(tk.END, file_name)
 
def get_all_audio_file_from_directory(self, directory_path):
  audio_files_in_directory = []
  for (dirpath, dirnames, filenames) in os.walk(directory_path):
    for audio_file in filenames:
      if audio_file.endswith(".mp3") or audio_file.endswith(".wav"):
        audio_files_in_directory.append(dirpath + "/"  + audio_file)
  return audio_files_in_directory