title: ffmpeg concat converts multiple videos to chipmunk version with half the video silence
body: <p>i try to concat multiple videos to one video and add an background music to it.</p>
  
  <p>for some reason the background music is perfectly added to the output video but the audio of each part of the output is speed up to a chipmunk version of the video itself. this results in an output video of 7 minutes with about 5 minutes of silence since everything is so fast that all the audio finishes after about 2 minutes.</p>
  
  <p>my command is:</p>
  
  <p><code>ffmpeg -safe 0 -i videolist.ffconcat -i bg_loop.mp3 -y -filter_complex "[1:0]volume=0.3[a1];[0:a][a1]amix=inputs=2" -vcodec libx264 -r 25 -filter:v scale=w=1920:h=1080 -map 0:v:0 output.mp4</code></p>
  
  <p>i tried to remove the background music (since i wasn't able to loop it through the video i thought maybe that's the issue) and still.. all the audio of the video clips is still speed up resulting in chaotic audio at the beginning and silence at the end.</p>
  
  <p>my video list looks like this:</p>
  
  <pre><code>ffconcat version 1.0
  file intro.mp4
  file clip-x.mp4
  file clip-y.mp4
  file clip-x.mp4
  file clip-y.mp4
  [... and so on]
  </code></pre>
  
  <p>i hope somebody can tell me what i'm doing wrong here (and maybe how to adjust my command to loop the background music through all the clips)</p>
  
  <p>i googled a bit and found the adjustment of my command to add <code>amix=inputs=2:duration=first</code> but that doesn't do the trick and if i add <code>duration=shortest</code> or <code>duration=longest</code> nothing changes the output audio</p>