The “use audio from layer” option is only available under specific conditions.
1) the input and output movies must both be AVI
2) the output movie must be the same number of frames as the input movie.
A much better way of working would be to work in pI, then when everything is perfect, turn off the bg image (using the BG icon in the layer window), render to an AVI format that supports alpha channel (like HuffYUV or Lagarith), check the “save alpha” option and render.
You’ll end up with a movie of just the particles. Then load that into your video editing software and layer it on top of your original video.
Make sense?
Alan.
http://www.wondertouch.com