Try this - something tells me that his is a bug.
Step 1: Open Neolemmix player (I have been using v12.10.2 for this) and go to 'Level Select' or if on level preview screen or level end screen, click 'Select Level'.
Step 2: Pick a level and select 'OK' on level select window.
Step 3: On level preview screen, click 'Select Level'.
Step 4: Pick a different level and click 'Cancel' on the level select window.
Step 5: On level preview screen, click 'Continue'.
If you follow these steps, the level that you play is the level that you select from step 4 instead of playing the level that you select in step 2 including swapping out the music to play the 'original lemmings equivalent' instead of the playing custom music except within the same pack.
If you also complete the swapped level, it marks the level you select in step 2 as 'solved'.
If attempting to 'read' a talisman on the level preview screen on step 5 before continuing will throw an 'argument out of range' error. It is also possible to collect talismans via the swapped level but only within the same level pack.
