These are a few changes that come to mind. I’m pretty sure these weren’t in any previous mix but if I’m wrong someone can correct me. It’s actually been a while since I last properly heard the 1993 mix.
One of the flashes that lacked a sound effect during the opening shot now has a sound (may have been in the mono), spark noises when the Jawa attaches the restraining bolt to R2 (also maybe from the mono), some laser-like noises added when they come out hyperspace into the remains of Alderaan (right when and after Han says “what the…?”), TIE Fighter laser sounds added to the blasts just before the fighter passes over the Millennium Falcon (before Luke says “it followed us”, I may be confusing this one with the SE), the aforementioned glass noises in the detention block shoot out, different sound to the laserbolt that shoots past Luke after Han says “either I’m gonna kill her or I’m beginning to like her”.
These I know were in or originated from the mono mix but added to the 1993 mix. First there are some additional rumble heard outside the door of the Tantive IV as the rebel soldiers await the stormtroopers (also in the SE). Second the loud mechanized noise heard in the compactor just before Luke resurfaces the final time is hear twice, once about a second or two before the second time but at a lower volume. All other stereo mixes including the SE (to my knowledge) use the effect once.
I also think this may have been the first mix to use a different sound of the explosion of the TIE Fighter shot down by the Millennium Falcon but I could be wrong.