In the scene where Ben& Luke are stopped at the Mos Eisley checkpoint, you can use the troopers voice from the 2004 DVD. The timing and the voice is exactly the same as the mono mix, whereas i'm pretty sure that on all the other mixes (LD, VHS etc) the troopers voice is timed different.

Also the original Tantive klaxon can be heard during the Death Star battle where Vader & the TIE pilots head for the ships. In the German 5.1 track the rear channels have the klaxon with no music so you could mix this (using loops) with the centre channel from the 2004 DVD and add music & ambient FX to recreate the mono mix. Takes a bit or work but it can be done.