The documentations (Appendix/Units/Spells/Editor) had a song playing when open that is not included in the OST. It is a simpler version (read: not over-edited and therefore better) of the Ambience 8 from the WBC1 OST.

It should be included and should be easily extractable.

I've never heard of this before. Is it accessible in-game, or is it via another exe?

He means the song that plays in the WBC2 Documentation. It is the same used by Dal Gurak's Unit Editor.