title: Add Audio to video with existing audio using FFmpeg and change volume of of both audios
body: <p>Add  Audio to video with existing audio using FFmpeg and change volume of both audios</p>
  
  <p>I am using this command from ffmpeg to merge audio to video</p>
  
  <p>video.mp4 has <code>video and audio</code>, and audio.m4a only has <code>audio</code></p>
  
  <p><code>ffmpeg -i video.mp4 -i audio.m4a -map 0:v -map 1:a -c copy -shortest output.mp4</code></p>
  
  <p>I want to merge audio to video but maintain audio from both video and the new audio file</p>
  
  <p>I also want to change the volume of audio file that I want to add from current file to 1.5 times </p>
  
  <p>and change the audio from the video file to 0.5 times</p>
  
  <p>how can we change the volume and still maintain new and old audio in the merged file</p>