I just listened to the original .wav scans, and compared to the ROTJ audio on YouTube at the spot you mention.
The clearest difference by my ears is right at the spot when Jabba pulls back Leia into his lap. On Youtube the chain sound is clear and loud, but on the optical scan there is barely any sound at all.
Possessed is right that it could be an artifact of the mono pulldown. Would need something a bit more obvious. Thanks CatBus, for listening so carefully.