title: No audio on concatenating clips in MoviePy
body: <p>I am trying to concatenate two clips using <code>MoviePy [ Windows 10 , Python 3.7.4 ]</code> , but there is no audio in the output video. I can see the temporary audio file while the videos are being concatenated.</p>
  
  <pre><code>from moviepy.editor import VideoFileClip, concatenate_videoclips
  
  clip1 = VideoFileClip("C1.mp4")
  clip2 = VideoFileClip("C2.mp4")
  final_clip = concatenate_videoclips([clip1,clip2])
  final_clip.write_videofile("my_concatenation.mp4")
  </code></pre>
  
  <p>Terminal gives this ouput,</p>
  
  <pre><code>Moviepy - Building video my_concatenation.mp4.
  MoviePy - Writing audio in %s
  MoviePy - Done.
  Moviepy - Writing video my_concatenation.mp4
  
  Moviepy - Done !
  Moviepy - video ready my_concatenation.mp4
  </code></pre>
  
  <p>I have also tried this <a href="https://stackoverflow.com/questions/54722792/output-video-has-no-sound">answer</a> but it doesn't solve the issue. Any ideas why this might be happening?</p>