I'm following the documentation for the Node.JS implementation of the IBM Watson Text-to-Speech API.

I want to output the resultant file into MP3 format. The documentation recommends augmenting the base code but I'm not sure how to do that. My code is rendering unplayable MP3s.

Here is what it says in the documentation:

textToSpeech.synthesize(synthesizeParams)
  .then(response => {

    // The following line is necessary only for
    // wav formats; otherwise, `response.result`
    // can be directly piped to a file.

    return textToSpeech.repairWavHeaderStream(response.result);
  })
  .then(buffer => {
    fs.writeFileSync('hello_world.wav', buffer);
  })
  .catch(err => {
    console.log('error:', err);
  });

As it says, response.result should be directly piped to a file. This is one of my many attempts (that renders an error).

textToSpeech
  .synthesize(synthesizeParams)  
  .then(response => {
    fs.writeFileSync('Hello.mp3', response.result)
  })
  .catch(err => {
    console.log('error:', err)
  })

How can I output the text-to-speech input as an MP3?