I mentioned this in passing over on the X0 Project thread, but for what it's worth, I've got a transfer of the 70mm audio mix straight to cassette tape. My father was fortunate enough to have been an assistant manager of a theatre in 1977 and ripped the soundtrack directly from the projection equipment, through patch cables into a cassette deck, and onto two dual-sided 60 minute cassettes, which have been stored in plastic cases for the past 25 years.

I don't know how good of a source 25-year-old tape is though... when I played it recently I found it warbles a bit but the line "Blast it Biggs where are you" is indeed there and it seems to be an actual stereo recording (not downmixed to mono/upmixed to stereo). I'm sure someone with a background in audio might be able to do something with it but it definitely needs some work.