There is no 5.1 channel audio on any of the pre-SE releases. This project aims to be an accurate preservation of the pre-SE laserdiscs. So the main audio we want to feature would be the PCM stereo track from the DC. Personally, I'd like to have the mono track as an option as well just for comparison's sake. But the PCM stereo track, being uncompressed, would be the highest quality audio we could use.

The THX trailer you mention was not on the DC laserdiscs, or any other laserdisc release for that matter. My guess is that it was a fan created work, but I could be wrong. It's way too long anyway in my opinion, and it will not be on our DVD.