Just a thought... at the moment, when installing a user-created level pack, music, sound, styles, levels etc must be unpacked to different folders which can be a bit messy, and some people don't bother to unpack music and sound files at all.
Could there be a level pack installation tool within the NeoLemmix player which shows a list of all available level packs, the player simply selects the packs they want and the installation tool unpacks everything to the relevant folders? It could be a two-way thing: level creators could upload a pack via the same tool, and it would then be available for other users to download.
If this seems a bit too radical, then could NL be made able to read music, styles, etc from within a level pack folder, so that all the creator needs to do is include everything in one parent folder? On the player's end, NeoLemmix would always choose to look in that folder first before looking in default folders for any music, spritesets, styles, or any customisable element.
Not sure if either of these things are possible/feasible, but since the online styles download already exists and does a fine job of keeping everything easily organised, could a similar idea be extended to level packs themselves?