I'm confused, so you mean that not all the music is there with the OST swap? And it don't play for long?
I haven't played the entire game to give you a solid answer, but so far I've skimmed through the 1st and 2nd floors and so far only the main hall has the original theme (which plays as long as you're there). All the corridors and rooms I've walked through so far have zero music. In fact, even in Original OST mode, when you encounter zombies the same humming ambience sound of the remake's soundtrack commences.
Just to be clear, I've switched soundtracks after encountering a licker for the first time, then just ran through multiple halls and corridors. Zero music.
Haven't visited the library (but I'm pretty sure it's going to have the original theme because it's one of the few rooms which is large and doesn't branch out into corridors).
Same goes for save rooms (i.e. film development room) - didn't go there yet with original OST.