title: iOS - Mixing two audio files
body: <p>I'm trying to mix 2 audios tracks into one (Video's sound and some music). </p>
  
  <p>With AVFoundation I can set two tracks to a video file with this method :</p>
  
  <p><a href="https://stackoverflow.com/a/16316985/5120292">https://stackoverflow.com/a/16316985/5120292</a></p>
  
  <p>Here is the result output file, inspected with ffmpeg on my computer :</p>
  
  <pre><code>Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'IMG_3789.mp4':
    Metadata:
      major_brand     : mp42
      minor_version   : 1
      compatible_brands: mp41mp42isom
      creation_time   : 2017-05-11T15:46:52.000000Z
    Duration: 00:00:22.63, start: 0.000000, bitrate: 38210 kb/s
      Stream #0:0(und): Video: h264 (Baseline) (avc1 / 0x31637661), yuv420p, 1696x848, 37782 kb/s, 29.97 fps, 29.97 tbr, 90k tbn, 180k tbc (default)
      Metadata:
        creation_time   : 2017-05-11T15:46:52.000000Z
        handler_name    : Core Media Video
      Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 250 kb/s (default)
      Metadata:
        creation_time   : 2017-05-11T15:46:52.000000Z
        handler_name    : Core Media Audio
      Stream #0:2(und): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 189 kb/s (default)
      Metadata:
        creation_time   : 2017-05-11T15:46:52.000000Z
        handler_name    : Core Media Audio 
  </code></pre>
  
  <p>You can see that the file contains two audio tracks. I would need to merge the two tracks into one, because some players only play the first track.</p>
  
  <p>Is it possible to mix them into only one track instead of 2 separated tracks ? </p>
  
  <p>Thanks in advance ! Any help would be appreciated</p>