What is the difference between the 5.1 1980 mix and the 2.0 1980 mix? Are there any content differences, or are they just mixed through more channels?
Also, does the 16mm mono track differ from those two in any way?
IIRC, the 5.1 1980 mix is just the matrixed stereo upmixed to 5.1, with some of the LFE channel from other sources like the Blu-rays where they still seem decent. In other words, it’s a nice, tasteful upmix of the stereo mix. Unlike Star Wars, it is NOT an attempt to reproduce the original six-channel mix. And unlike Jedi, we know that there are content differences from the six-channel mix, but the six-channel mix didn’t apply to this cut of the film. So no content differences, just more channels, technically not theatrical but not actively revisionist in any way.
The 16mm track is way different. C-3PO gets some different lines (ones that later showed up in the Special Editions), and the mixing is occasionally a bit different too.
So in other words the 5.1 is just stereo? It does sound amazing but just interested as i thought it was a pure 5.1 source. Is there any interest in obtaining a true 5.1 source or using the 6.1 source from the blu rays etc?
No, it’s an upmix of matrixed stereo, not plain stereo, which gives you 4 distinct channels. Mix in the LFE and it’s true 4.1, upmixed to 5.1. The channels aren’t as distinct as a modern discrete mix but they’re distinct enough. However, it is just the stereo mix, i.e. sound effects and dialogue specific to the stereo mix. No content from the original mono or six-channel mixes.
The point of the 1980 six-channel mix is to be authentic yet multichannel. You could certainly get more channels from the Blu-ray, but you’d lose everything else–quality, authenticity, etc. 4.1/5.1 is about as good as you can expect in this regards when the cadillac best-of-breed audio in 1980 was 4.2. The only thing theoretically better would be a preservation of the 70mm version of the video with accompanying six-channel audio, but that’s unlikely–and frankly I don’t like the 70mm cut as well, so what we have now is still better, IMO.
It is possible to insert 6.1 sound from the Blu Ray to the Despecialized edition, however the Blu Ray is the special edition cut with added shots not seen in the Despecialized edition.
Also the audio quality on the Blu-rays is fairly tragic, so if you can find a nice, alternative lossless source (which we have), you’ll end up with considerably better results than anything using the Blu-rays as a significant source. You’re not going to improve anything by pulling in audio from an inferior source, regardless of the nominal channels.