I remember reading this a while ago, I don't think I saw it posted here:

WSR Sun: Did you work with the stems, or just the final mix?

Rydstrom: I worked from the final mix stems, so I had the dialogue, music and effects separate and was able to do some remixing from those. I did some re-balancing between those elements, but mostly what I was doing was re-positioning some of the effects in the surround channel, placing them into the back surround channel and shifting some content from the front to the surrounds. I also generated surround channels for the music. The music actually consisted of only the front channels, and when we did the original mix, we generated a surround channel for the 70mm CDS version with the Dolby Stereo matrix encoder, which gave us just mono surround for the music. [CDS is Cinema Digital Sound, a short-lived digital film sound format.WSR Sun] So for this remixing, I did a pass where I would generate, through reverb units and the like, stereo reverb surround channels for the music.

http://www.widescreenreview.com/news_exdetail.php?title=vanling