Lt/Rt matrix audio decode

I'm working on two small projects that have been previously mixed in 5.1 descreet surround with a two track Lt/Rt dolby matrix pro-logic mix. I know the matrix is supposed to stay with the two track Lt/Rt unless intentionally mono'd, but I can't seem to find any indication of it anywhere in FCP or STP. I've pulled it into compressor, but it doesn't detect it. I'm hoping to make a small change to each of the pictures and want to be able to output a DVD with the Lt/Rt track, so when played through an amp that can decode it, it produces the desired surround. I'm changing the end credit music on one of the pictures, due to a licensing despute and I'd like to do this without going on a digidesign pro-tools system.

Any discussion would be appreciated. P.S. I'm working from FCP 5.04 and the corresponding STP

Since I wrote this originally, I've spoken with some techs at a local video house.
Apparently if you alter Dobly Digital AC-3 files in any way, they become simply stereo files. The Dolby encoding is negated. They said that you have to decode the surround to seperate channels and re-encode it in compressor for DVD Studio Pro.

Barrett Taylor