They actually released an Amiga demo disk with the "60's/70's action music" [1] hidden away on it. None of those tunes played when you played the demo, of course.

If you're interested in hearing them, I reverse engineered [2] the music format a while back and converted them to ProTracker modules [3].

[1] https://tcrf.net/Proto:Lemmings_%28Amiga%29#Unused_Music [2] https://bitbucket.org/mindless/sjstomod/src [3] http://www.camanis.net/lemmings/files/rips/music/lemmings_de...