It's a known issue that stems from the fact that most GOUT-based projects use the PAL version of ROTJ because it is a native PAL master unlike the other two films which are converted from NTSC. But these projects use the NTSC sound files to avoid additional processing on the sound. Because it's a different master, the frames aren't exactly accurate and in this case there is an extra frame or so when the fleet reassembles to discuss their attack on the Death Star 2 and there is a frame missing later on.... So in between these two points, the sound is out of sync.

I think it was catbus who made a corrected version and I have this muxed with my personal file.