title: Add overlay but output has wrong audio
body: <pre><code>ffmpeg -i Knight.mp4 -i "4h.mp4" -filter_complex "scale=854:480 [v1]; [1:v][v1]overlay=20:20" output.mp4
  </code></pre>
  
  <p>But <code>output.mp4</code> not have <code>Knight.mp4</code> audio.</p>