Uses sox cmdline tool, to convert audio file to .wav

        Note: for testing, use -
        http://www.text2speech.org/,
        with American Male 2 for best results

def convert_to_wav(self, filename):
        """
        Uses sox cmdline tool, to convert audio file to .wav

        Note: for testing, use -
        http://www.text2speech.org/,
        with American Male 2 for best results
        """
        temp_filename = '{0}.wav'.format(self.temp_filename())
        self.run(['sox', '-G', '-c', '1', filename, temp_filename])
        return temp_filename