title: merge png with mp4 ffmpeg missing audio
body: <p>I use ffmpeg to merge mp4 and png, I use two way:</p>
  
  <ol>
  <li><p>use command  </p>
  
  <p>String cmd = "-y -i " + in.mp4 + " -i " + in.png + " -filter_complex [0:v][1:v]overlay=0:0[out] -preset veryfast -map [out] -map 1:0 -map 0:0 -codec:a copy " + out.mp4;</p></li>
  </ol>
  
  <p><strong>output file missing audio:</strong></p>
  
  <ol start="2">
  <li><p>use command:</p>
  
  <p>String cmd = "-y -i " + in.mp4 + " -i " + in.png + " -filter_complex [0:v][1:v]overlay=0:0[out] -preset veryfast -map [out] -map 0:a -codec:a copy " + out.mp4;</p></li>
  </ol>
  
  <p>=> There is audio but some mp4 file cannot merge with png file
  Log: Stream map '0:a' matches no streams.</p>
  
  <p>What is my command missing here ? </p>