Jonno said:

I've had a more extended listen to the three tracks on the Blu. Unfortunately the additional Kane lines you mention are also included on the 4.1... more evidence that it isn't the 70mm, or at least not the original 70mm. More detail from this short 2002 thread on AVS:

The problem with the
current DVD vs. the THX Certified Dolby AC-3 RF LaserDisc is not only
one of bass, but the fact that two different masters were used.

The LD's Dolby Digital track is encoded using the 70mm 6-track master.
It does contain sound cue and music cue errors that Ridley Scott wanted
fixed, however. It was one of those rare 6 track mixes that was the
model for today's 5.1 surround (another was Apocalypse Now's 70mm
track): three front channels, two stereo surround channels, and a "baby
boom" channel.

Notice I did say stereo surrounds.

Which brings me to the DVD we have at the moment. It has the sound
corrections Ridley Scott wanted. Now here comes the problem: they
didn't fix the 70mm 6-track master, they mistakenly chose the 4 channel
Dolby Stereo printmaster to correct. Meaning, no low bass "rumble"
track and mono surrounds.

...

Dan

It is of course possible that the errors, i.e. missing lines, from the original 70mm mix were reinserted for this Blu presentation. And it's otherwise a strong sounding track - impressive bass on the Nostromo landing sequence, particularly in comparison to the 2.0 surround track (which presumably comes from the 35mm stereo mix).

I guess that's a possible explanation for not hearing the dialogue differences but it's a little odd, it kind of defeats the purpose of including the track on the Blu-ray IMO. I don't know, I can understand that Ridley Scott wants to correct certain things for a remix, but that's what the remix on the disc is intended for. If that's the case, I'm a little disappointed.

That guy also mentioned stereo surrounds, was the 70mm mix on Alien really presented with stereo surrounds? Maybe that's impossible for you to remember, according to http://www.in70mm.com/library/process/dolby/index.htm it wasn't. (I guess SS - means split surround or stereo surrounds)

Jonno said:

Bottom line - I'd really like to have a listen to that laserdisc track! I have neither a laserdisc player, the disc itself nor the necessary PC tech - there are folks on this thread discussing capture of digital streams from laser, so there may be some ideas there as to how we might organise a preservation. Does your friend have the means to do any sort of capture, M, even if it's a little basic?

I would also love to have it preserved, the two tracks on the Laserdisc is in 5.1 AC-3 and 2.0 PCM. I wonder which of them is the best sounding, would be great to have both preserved. I don't think he have the right equipment for making a good audio capture, I can of course easily make a basic recording of the PCM track, it would be better than nothing but it would be nice to have something better than that. I'll have to check out what's needed for this, I do own a demodulator for the AC-3 track at least.