This apparently doesn't really differ between IN, HTTT and DW, though I may be wrong.
Whatever, I know you can add music by putting MP3 in the Music folder. What seems weird to me is that many tracks seem to never play, and others to play all the time.
Or to put it simply: I have the feeling only something like 30 music tracks can play in EU3, as if the game picked them all the time - either because their first in alphabetical order, or because they're latest additions into the folder, or last created music files (or some other weird ranking I can't deduce).
Is it just me that doesn't notice Falalalalan anymore (since I put EU2 music into the EU3 folder), but just Bach, or is it something well-known (as is probably the case), that the game doesn't play your full playlist if you put 200+ MP3 files in the Music folder?