What JediTemple said.
The 50-gig capacity of dual-layered blu-ray discs would allow them to include all of the sound mixes.
here's an interesting breakdown of the differences in the sound mixes:
http://starwars.wikia.com/wiki/List_of_changes_in_Star_Wars_re-releases#Theatrical_Releases
I'm assuming there were no differences for ROTJ since it's not listed. It was, after all, the first THX-certified theatrical release.