title: Problem when adding background music to video without sound
body: <p>I am trying to cut a piece of background music and loop them into a video and change the speed of the video with the command below.</p>
  
  <pre><code> String[] cmd = {"-y", "-i", input, "-i", shortsound,
              "-filter_complex",
              "[0:v]trim=0:120,setpts=1/2*(PTS-STARTPTS)[v1]," +
                      "[0:v]trim=120:240,setpts=1/2*(PTS-STARTPTS)[v2];" +
                      "[1:a]atrim=0:6,asetpts=PTS-STARTPTS," +
                      "asetrate=44100,aloop=-1:2e+09," +
                      "aformat=sample_fmts=fltp:sample_rates=44100:channel_layouts=stereo,volume=1.0[bg];" +
                      "[0:a]atrim=0:120,asetpts=PTS-STARTPTS,atempo=2.0" +
                      ",aformat=sample_fmts=fltp:sample_rates=44100:channel_layouts=stereo[a1]," +
                      "[0:a]atrim=120:240,asetpts=PTS-STARTPTS,atempo=2.0," +
                      "aformat=sample_fmts=fltp:sample_rates=44100:channel_layouts=stereo[a2];" +
                      "[v1][a1][v2][a2]concat=n=2:v=1:a=1[video][audio];[audio]volume=0.5[avolume];" +
                      "[avolume][bg]amerge=2,pan=stereo|c0&lt;c0+c2|c1&lt;c1+c3[aout]",
              "-map", "[video]", "-map", "[aout]", "-c:v", "libx264", "-c:a", "aac", "-shortest",
              "-preset", "veryfast", output, "-hide_banner"};
  </code></pre>
  
  <p>Everything was fine until I tried with a video without sound, it didn't work.
  How when input is a video without sound?</p>