Right before the Imperial walker blows up the main power station, it shoots down a speeder.
That speeder's out-of-control engine sound before its crash was mysteriously removed from the '93 remix.
It's heard on Digital Sound laserdiscs of the late 80's as well as analog LDs from the mid-80's.
I've always assumed Burtt couldn't resist his urge to tweak the mix when he made the first digital master in the mid-80's (like he did Threepio's tractor beam line in the first film). Maybe I'm wrong. It could be true that all pre-93 versions, both analog and digital, had the same content possibly with different EQ or dynamic range compression.
Despite some difference(s) like the speeder engine sound, all pre-SE versions (both pre/post-93) seem to line up in the "35mm" right-hand column of the list Arnie.d posted from wookieepedia.
Not until SE'97 that we heard those 70mm elements for the first time in an official home video release.