[tinybilbo]

that completely depends on the trailer source you have chosen. but in short: none of the trailer sources support downloading out of the box;
the worst is youtube which offers not much muxed trailers (audio and video in one stream) - so we have to take what we get; muxing the audio into the video stream is not possible at the moment
apple trailers should be the best at the moment since they are not hiding their video files as good as youtube does..

you may have success with choosing apple as the preferred trailer source (along with hd-trailers.net)

Thank you, I'll try that :-)