Oddly enough, there is ONE visual difference. As the stormtroopers
are distracted by the duel between Vader and Kenobi, Threepio turns
and says, "Come on, Artoo. We're going." CUT to Han who says, "Now's
our chance, go!" In the version with the mono mix, these two shots are
reversed!
http://www.cedmagic.com/featured/star-wars-lost-footage.html