OK, I've just tried it, and as you suspected, the order of the songs isn't right for ONML. I also tried in regular Lemmings with the ONML Adlib.dat to see where the songs fell. I don't know the order of the ONML songs, since I never was able to play ONML with music (had to just play it in XP), but the locations of the songs fell as follows:
Level 1
Level 6
Level 11(I think?)
"A Beast of a level"
"What an AWESOME level"
"A BeastII of a level"
Perhaps the author(s) of CustLemm hacked the ONML music code to read tracks from original Lemmings? Is it possible to change what tracks are pointed to? (Based on what you said about even putting this data into a level, I'm guessing this is possible.)
Side question: Just curious, does your way of putting the music data inside a level have any connection with how the special levels point to their own songs?
I don't know about what anyone else thinks, but it doesn't seem that the ONML songs are nearly as good as the original Lemmings ones. I can't imagine listening to those 6 songs through 100 levels! A couple of them are not bad, but some of them are grating. A0;X_X
By the way, ccexplore didn't mention this, and it may seem obvious, but when playing "nocdlem.exe" in DOSBox, you don't need to mount your CD-ROM drive.