Where are you getting your information about the lost cut from? Sorry, but it’s completely wrong. There was no sound mix done for the Lost cut. It was mainly on set audio but also silent in a lot of places. The cut was put together with what footage was available at the time (1976). There were only a handful of FX shots in the cut, with the ones that were being mostly unfinished or rough versions. They hadn’t even finished shooting the live action stuff, so many shots were just slates where a shot needed to be inserted later. The opening scene had NO laser fire or flashes added until the final shot where it was hit. Footage from the B&W lost cut has been released ( The cantina scene and a section of the opening battle, Luke & the Treadwell scene that’s in B&W) including the whole Tantive IV chase), so we know exactly what the opening of the lost cut looked like.