Just to clarify the audio mixes, the original theatrical version of Alien had a pattern that somewhat reflects that of Star Wars - a 6-track 70mm mix for initial engagements, and a more refined Dolby Stereo track for the 35mm prints (I've also read claims that it was screened in mono, but since I can't find any evidence that it was ever mixed that way I presume it was just a case of the stereo track playing in mono-equipped screens).

The 70mm track has only ever been released on home video once - on the 1995 THX laserdisc (the possibility that it might have been included on the Blu-ray as a 4.1 track was under heavy discussion, but in fact its split surrounds would have necessitated a 5.1 encode so that was never going to be the case). We've been able to listen to this track via PCM captures for a while, but Andrea has finally captured the 5.1 AC-3 (and it does indeed sound magnificent).

The Dolby Stereo is much better known, and it's this that is present on the Blu-ray as a discrete 4.1 upmix (actually Fox have a welcome tendency to do this - see also Predator - but sadly they always opt for a lossy Dolby Digital encode). The preferred source for this mix is an earlier generation laserdisc (since aside from lossy compression the track has also had some 'improvements' made since 1999). I favour the 1992 collector's edition but there are others.

When the director's cut was put together in 2003 it was given a theatrical release, so the studio took the time to do a proper remix. The DC had a DTS mix in theatres and subsequent home video releases (DVD, D-Theater and Blu-ray) have more-or-less preserved it, though access to the theatrical mix is very welcome for a reconstruction project.