Moth3r said:

What are the minor differences between the 35mm and the 70mm audio? I seem to remember something about a missing effect during the snowspeeder crash.

Am I correct in thinking that the list of audio differences in the wikipedia article is incorrect and actually represents differences between the theatrical and SE mix?

Yes, the wikipedia article on this is totally wrong. The changes it lists are just differences for the SE, and have nothing to do with the 70mm version. For a long time, since there was no recording of the 70 for anyone to check against, it was often assumed that it must have contained many of these SE differences; but once the in-theater recording surfaced, this rumor was proved to be incorrect. In actuality, it is very nearly identical to the 35mm version. In addition to the in-theater recording, morgands1 also provided a link to an article from 1981 where the mixing of The Empire Strikes Back is specifically discussed, in which the mixers say outright the two were very close, and that very few changes were made for the 35.

That article can be found here: http://www.in70mm.com/news/2015/mixing/index.htm

Most of the differences between the 70 and the 35 are so small as to be virtually inconsequential. Here is a complete list from what I remember:

The sound of Luke’s lightsaber deactivating as he leaves the Wampa cave is clearly audible in the 70, while in the 35 it is greatly reduced in level. This was probably changed because he is far from the camera and moving away.
When the walkers first open fire in the snow battle, the explosions are significantly louder in the 35.
One of the snowspeeder attacks on an Imperial walker uses a laser sound normally used for TIE fighters. In the 35 this was changed to the X-wing laser sound, which the speeders use everywhere else in the snow battle.
When 3PO says “R2, you take good care of Master Luke now, understand? And . . . do take good care of yourself,” the word ‘and’ is clearly audible. In the 35, the ‘and’ has been reduced significantly in level. (Not sure of the reasoning behind this one; perhaps it was to make him sound less hesitant in telling R2 to take care.)
In the transition from Dagobah to the Imperial fleet after the cave scene, a different TIE fighter sound effect is heard.
The end scene is edited completely differently. It does not contain tracked music from earlier in the film, as it does in the 35, due to the scene being shorter. Lando’s line “When we find Jabba the Hutt and that bounty hunter, we’ll contact you,” is absent. His lines “Luke, we’re ready for takeoff,” and “Princess, we’ll find Han. I promise,” are different takes.

That’s it. There are no other changes in content between the 70mm and 35mm mixes of Empire. The re-editing of the last scene, which was mainly done to accommodate the insertion of additional shots of the rebel ships, is by far the largest change, and the only one that is particularly significant. While it might be possible to recreate the other changes by editing in bits of other mixes, the end scene would be difficult. Perhaps not impossible, but there wouldn’t be any point unless the picture were also edited to match the difference in length; and there wouldn’t be any point in that unless the other visual differences could also be reproduced.

The 8mm version, contrary to what was stated earlier, is not at all the same as the 70mm. While it does use a number of the early visual composites created for the 70 which were replaced in the 35, the audio track is entirely separate, and its numerous differences in ADR are not present in the 70mm mix.

The snowspeeder crash sound being missing in the 1993 mix is an odd error that doesn’t appear in any other version. Both the 35mm and 70mm mixes do have it. My theory is that the four-track master omitted it mistakenly, and that it wasn’t corrected on that copy because only parts of the mix would have been re-recorded for later changes. Therefore, this early mistake ended up on home video even though it had already been fixed for the theatrical releases. That’s the danger of going back to earlier generation copies: better sound quality is obtained, but any later changes may not be present.