title: How change audio volumes with amix filter in ffmpeg
body: <p>I have command to mix two mp3</p>
  
  <pre><code>ffmpeg -i 1.mp3 -i 2.mp3 -filter_complex amix=inputs=2 -c:a libfdk_aac output.mp4 
  </code></pre>
  
  <p>How to change volume of every audios in result output?</p>