*Luke steps off the platform where the carbon freezing happens during his duel with Vader. Instead of just stepping or jumping down, or whatever he does, insert a Wilhelm scream and loud crashing and thumping as he falls a very long distance, accompanied by shouts of pain. You could also cut to the shot of him falling into the big shaft.
*When Luke falls off the antenna thing into the Falcon, he should land with a loud thump and shout in pain (it’s always looked to me like he just lands on his head rather than falling into the Falcon).
*If possible, every time Luke says “Vader” or “my father” or “Father” he should say “Daddy” or “my daddy” instead, once he finds out who Vader is.