the original mono mix can only be found on the old us laserdisc and japan laserdisc. the mono audio on these ld is also analog. so to get the best quality, someone with a good capture setup would be need to capture the audio.
everything after these laserdisc had the circle surround remix, up to the dreadful 7.1 remix on blu-ray. at least they included the original mono mix as a lossy dolby track.