Status: Fix will be included in next update.
Even when I rename the Acorn Lemmings music pack to NeoLemmix_Music.dat, when I play a single level, it plays the Amiga music instead of the Acorn one. This does not occur in test mode of the editor though; only when you directly load the single level from the EXE.
I'll look into this later today. I have an idea of what might be causing it.
Found the cause and fixed it for the next update.