title: ffmpeg genereated mp4 have no sound
body: <p>I have a mp4 file with two audio channel,as below.<a href="https://i.stack.imgur.com/HjaSW.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/HjaSW.png" alt="enter image description here"></a></p>
  
  <p>Then I use <code>ffmpeg -i 70_1.mp4 -ss 00:00:00 -t 00:00:10 -hide_banner 777.mp4
  </code> to cut a slice, but the generated file lost sound.</p>
  
  <p><a href="https://i.stack.imgur.com/Wcvar.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Wcvar.png" alt="enter image description here"></a></p>