title: adding silent audio to channels 3-8 in ffmpeg
body: <p>This has been discussed before, but my question is a bit different.
  Lets say have a video file with 1+2 as stereo.
  Now I want to add 3-8 with silent audio - most likely using the anullsrc.
  I need to map that generator to only affect channel 3-8.
  Anyone has a solution for that?</p>