I don't see any reason (Audacity at least) couldn't put the center and bass tracks in the downmix. In fact, according to that article, that's probably all you'd want, since usually (but not always) the rear surround tracks are just echo and stuff. I'm no audio engineer, I just want to hear the 5.1 version of Still Life on my headphones.
The "5.1" mix on the Still Life DVD did not have a bass track. The center channel is mostly vocals and some drums, but interestingly some guitar was only on what I assume are the surround tracks. So for this one I used all channels in the mix. First I used the amplify effect to reduce all channels 6db. (When I omitted this step, the downmix levels got raised (due to channels coming together), and there was clipping.) I could have probably reduced it less since the result was still a ways off from peak, but I didn't want to do it again. Anyway, the result seems pretty good, but it's hard to compare. At least this version isn't as hot as the original (the remix is even worse), and doesn't have the channel dropouts on The Moor. But I'm sure it could be done better. The article said to make the center track stereo and reduce it more so it would go equally into the left and right. However, I left it and the other two as mono. I'm not exactly sure how Audacity balances those stereo-wise, but I know it included them because the downmix includes some guitar parts that aren't in the original left and right tracks, so I know they came from the surrounds.
The other disc I was playing with was David Gilmour - Remember That Night. That did have a bass track, and the rear channels seemed more surround-y that music-y. I might have to do that one differently. The 2-ch mix on it is good anyway, but I had it handy.
