Basically what it comes down to is that there's special types of levels that NeoLemmix supports, called "Frenzy" and "Gimmick" levels. The former disables pausing; the latter adds twists to the mechanics. On these levels, different musics play; additionally, SuperLemming levels also play the Frenzy music. Since regular Lemmix doesn't support such features, the musics for such aren't included in its music packs. Thus, when you try to play Wicked 2 (which is a SuperLemming level, as the name suggests) with a regular Lemmix music pack, it crashes because it can't find the Frenzy music.
(The "Extra levels" player, I think the musics are also in a slightly different order in the regular Lemmix version than they are in the NeoLemmix version, so while it'll work, you might get special graphics level musics on regular levels and vice versa.)