Trooperman said:

Lando walking down the hallway. His line: "So you see..." is clearly audible in the mono, not in the stereo (although it is there, faintly).

^ Some reverb seems to have been added to their voices in that sequence as well, listen to Lando's voice when they're about to walk around the corner in the hallway down the stairs. Some added Chewbacca vocalization short after that, heard over the shot when Boba Fett makes his entrance after Vader's "We would be honored if you would join us."