title: How to merge mp4 audio dash fragment with another audio
body: <p>I have 1 audio file from dash stream </p>
  
  <pre><code> Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'file_191282-377206_header.mp4':   Metadata:
  
   major_brand     : iso6
   minor_version   : 1
   compatible_brands: mp42dashmsdhmsixiso6avc1isom
   creation_time   : 2016-04-29T11:04:26.000000Z   Duration: 00:00:30.02, start: 14.997333, bitrate: 49 kb/s
   Stream #0:0(eng): Audio: aac (LC) (mp4a / 0x6134706D), 48000 Hz, stereo, fltp, 48 kb/s (default)
  </code></pre>
  
  <p>Also in exiftool output:</p>
  
  <pre><code> Movie Data Size                 : 180193
   Movie Data Offset               : 6388
  </code></pre>
  
  <p>I try to merge with another audio and save metadata info like Movie Data Size/Offset and start/duration time. 
  I try with ffmpeg/MP4Box commands like:</p>
  
  <pre><code>MP4Box -new -add file_191282-377206_header.mp4 -add out000.mp4
  ffmpeg -i file_191282-377206_header.mp4 -i out000.mp4 -codec copy -shortest output.mp4
  ffmpeg -i file_191282-377206_header.mp4 -i out000.mp4 -filter_complex amerge -ac 2 -c:a aac output.mp4
  </code></pre>
  
  <p>All the time information erased or changed.</p>
  
  <p>So question is how to merge 2 audio files inside mp4 and not change Movie Size/Offset and start/duration time?</p>