title: Download video in mp3 format using pytube
body: <p>I have been using pytube to download youtube videos in python. So far I have been able to download in mp4 format. </p>
  
  <pre><code>yt = pytube.YouTube("https://www.youtube.com/watch?v=WH7xsW5Os10")
  
  vids= yt.streams.all()
  for i in range(len(vids)):
      print(i,'. ',vids[i])
  
  vnum = int(input("Enter vid num: "))
  vids[vnum].download(r"C:\YTDownloads")
  print('done')
  </code></pre>
  
  <p>I managed to download the 'audio' version, but it was in <code>.mp4</code> format. I did try to rename the extension to <code>.mp3</code>, and the audio played, but the application (Windows Media Player) stopped responding and it began to lag. </p>
  
  <p>How can I download the video as an audio file, in <code>.mp3</code> format <b>directly</b>? Please provide some code as I am new to working with this module.</p>