Acorn: https://www.youtube.com/watch?v=QwXthGJfHLc
Amiga (emulated): https://www.youtube.com/watch?v=ZT6vxY9p7yg
The Amiga's way brighter, but that's probably because it's emulated (and so is a digital recording). But it's pretty obvious that at least some of the tracks on the Arc are recompositions --- One Way Or Another is missing a lot of instruments, Mind the Step is a different pitch, Awesome is a different composition entirely...
The Amiga had a hardware mixer with four channels (with fixed stereo positions). The Acorn had a more strictly powerful CPU but had no hardware mixer. Maybe they had to reduce the number of channels to allow for this? But I was pretty sure the Lemmings mods were classic 4-channel anyway (one channel per Amiga hardware channel).