Just a default NeoLemmix installation contains something like 150 music files in the base "music" folder. While newer packs are starting to make use of proper organisation, it's still not as common as would be ideal (after all - ideal would be "every music file, except maybe the official tracks, is in a subfolder").
NeoLemmix player has supported the use of subfolders for music since new-formats came out. The editor didn't play nicely with it at first, but as of the latest version it works. I'm not sure if it displays them in the list yet (or if this feature is planned), but you can type it in and it works fine. For example: "Lemmings_Plus_V\abstract1". (It doesn't matter which way around the slash is, either will work.)
Packs and styles get sorted properly because NeoLemmix enforces it. I don't think it's practical to enforce this in the same way for music, given that some tracks are very much not pack-specific (like those from the official games, which are used across many fanmade packs), but it's still something content creators should take into account and make use of IMO - it keeps the music folder tidier, and also reduces the risk of ending up in a situation where two different packs conflict because they used the same filename for a music track.
(Side question: Does the same apply to sound effects? I don't know if NeoLemmix handles this correctly or not. Even if it does, perhaps there's less of an argument for sound effects - there's going to be far less custom sounds than custom music, and a much higher likelihood of reuse of a custom sound in another user's content.)
