def extract_audio(filename, channels=1, rate=16000):
    """
    Extract audio from an input file to a temporary WAV file.
    """
    temp = tempfile.NamedTemporaryFile(suffix='.wav', delete=False)
    if not os.path.isfile(filename):
        print("The given file does not exist: {}".format(filename))
        raise Exception("Invalid filepath: {}".format(filename))
    if not which("ffmpeg"):
        print("ffmpeg: Executable not found on machine.")
        raise Exception("Dependency not found: ffmpeg")
    command = ["ffmpeg", "-y", "-i", filename,
               "-ac", str(channels), "-ar", str(rate),
               "-loglevel", "error", temp.name]
    use_shell = True if os.name == "nt" else False
    subprocess.check_output(command, stdin=open(os.devnull), shell=use_shell)
    return temp.name, rate