import subprocess
subprocess.call(['sox', 'filename.mp3', '-e', 'mu-law',
                       '-r', '16k', 'filename.wav', 'remix', '1,2'])


import pydub
sound = pydub.AudioSegment.from_mp3("filename.mp3")
sound.export("filename.wav", format="wav")

RuntimeWarning: Couldn't find ffprobe or avprobe - defaulting to ffprobe, but may not work
      warn("Couldn't find ffprobe or avprobe - defaulting to ffprobe, but may not work", RuntimeWarning)

(RuntimeWarning: Couldn't find ffprobe or avprobe - defaulting to ffprobe, but may not work
      warn("Couldn't find ffprobe or avprobe - defaulting to ffprobe, but may not work",RuntimeWarning)