I was about to call rubbish on the dismissal of the mono mix, but several others have already done it for me, so I can tone down my reflexive outburst. ;)

As has been said, the mono mix sounds nothing at all like the other versions. It contains numerous differences in ADR, sound effects, and general balance that give it a completely different vibe than any of the others. The music is mixed much more prominently throughout the movie, and in some respects it seems more 'polished' and refined, since they had more time to work on it and to correct things that were perceived to be lacking before.

It will never be my favourite version, as I became quite attached to the sound of the Dolby Stereo as a kid and because the 70mm version is amazing and leaves everything else in the dust; and also because some of the things I didn't like about the SE mixes were actually things that first appeared in the mono mix, but I didn't know that yet at the time.

However, as a '77 original, considered 'definitive' when it was made (stereo and surround were still thought of as 'gimmicky' back then before they caught on), and being what many people would have heard in theatres at the time, the mono mix is completely worthy of preservation. It was never released on any home video format, only appearing on rare television broadcasts from the 80's and then never again. I am very grateful to Belbucus for restoring it so well for use in fan projects, so that it will live on even if, as seems likely, it never officially sees the light of day again.