There seems to be some confusion about the audio tracks . . . CatBus explained it well, so I’ll reiterate what he said to make sure it’s clear.
The Empire Strikes Back had two versions of the film released in theaters: the 70mm cut, which visually was about 99% complete and had a six-channel mix; and the more familiar 35mm cut, which had a two-channel mix and some small refinements to some effects and editing. The 70mm mix and the 35mm mix are both largely derived from a common source, the original four-track master tape, which had a channel arrangement of Left, Right, Center, and Surround channels. The 70mm mix contained those four channels exactly as they were, plus two tracks devoted solely to bass content. It is a powerful track with a lot of dynamic range and plenty of low frequency support. The stereo mix contains the original four tracks folded down into two, and is intended to be expanded back into four channels during playback with Dolby Prologic or something similar. Its dynamics and bass are reduced appropriately to fit into the technical limitations of optical film audio. There are a few very minor differences between the two versions, but nothing really noteworthy. For the most part they sound exactly the same in content and mixing balance.
Since the 70mm mix was only ever used with the earlier cut of the film, and every subsequent release used the completed 35mm edit, technically I could not call my 5.1 version a reproduction of the 70mm audio because it would not match up with the timing of the 35mm edit, and I don’t really have a way to duplicate some of the small changes it included. Therefore, I elected to make a multichannel version, with the full dynamics and bass which that entails, that matched up with the 35mm edit we actually have available for viewing. The best source for this was the 1993 laserdisc, which was actually derived from that same original four-track master the theatrical mixes came from. I converted it up to five channels so that I could bundle it with a custom LFE track (the .1) for additional bass. The result is virtually identical to what the 1993 version sounds like when it is upmixed in a receiver, but with a stronger low end presence. So while it isn’t an exact match for what was heard in theaters, it is nonetheless very authentic in its design. The 35mm cut never had a six-track mix made for it, but if it had, it probably would have sounded quite similar to what I did.
For those worried that the number of channels isn’t high enough compared to the Bluray, or that it isn’t surround-y enough, don’t fret. Like CatBus said, it isn’t just a regular stereo mix that’s been spread out; there is plenty of real surround sound going on. Just listen to the launch of the probe droids in the very beginning and you’ll hear them passing into the rear channels as they leave the Star Destroyer. There’s plenty of other things of that nature throughout the movie, and you can hear all kinds of ambient sounds on Dagobah in the surrounds as well. The rear channels are also better balanced in the original mix compared to the special edition: in the SE, they couldn’t resist the temptation to make the surrounds really loud and flashy, which is distracting and calls too much attention to itself in a bad way. The original mix had more seamless panning from front to back and was really well done. Everything blends together nicely, while the SE never achieves that level of cohesion.
In short, you’ll get an excellent aural experience with the 5.1, as well as plenty of authenticity. I’ll add my standard reminder that it only really sounds the way it should on an actual 5.1 system, and that when viewing the films on a stereo system, the stereo mix will give a better result. The version of that I chose from schorman’s archive was the 1989 Special Widescreen Edition laserdisc, which seems to be the most dynamic version of the stereo mix ever put on home video. It too sounds excellent, and I highly recommend it. If you don’t want lots of bass, you can upmix that and it gives just as good a surround experience as the 5.1 does.