From memory, mono mix has:

Different alarm klaxons on Tantive IV
Different dubbing for Aunt Beru (either a different actor or different takes)
Cantina music can be heard through the door before Luke and Ben enter
Searching troopers say “This one’s secure” instead of “Door’s locked” when Artoo and Threepio are hiding
Different sound effects when the Falcon is in Alderaan’s asteroid field
Different sound effects for the DS tractor beam
Threepio’s tractor beam line added
“Close the blast doors” added
Different echo effects in the chasm scene
Different radio effects for X-Wing pilots
“Blast it, Wedge, where are you” instead of “Biggs” during the Battle of Yavin

I know I’m missing a bunch, though.