Friends, I have a number of .mp4 videos encoded with 5.1 (6 Channel) AAC audio. I would like to end up with an .mp4 with AC3 5.1 and 2 channel AAC encoded for Dolby Prologic II (such as the way Handbrake will do when encoding a video using AC3 passthough). I do not need to re-encode the video. I stress that I don't want to reencode the video, only adding the AC3 5.1 track from the current AAC 5.1 track. If I hit the "ATV3 Specs" alternative in Handbrake every movie takes about 1.5h to re-encode (it re-encodes everything). Any ideas? Tried mkvtomp4 but it can only ac3 -> aac and not backwards. That's a shame because I love that program, and it's so fast.