Not that the in-game music is not worth listening to or something, but I'm curious to know what else you might play instead of the official tracks. Mainly I'm looking for dark and ambient (possibly classical) music that fits the game whilst searching around and investigating things or even for cut scenes, but not necessarily for the action sequences.
Some artists I currently listen to are:
Atrium Carceri (Cellblock, Souyan, Kapnobatai, Reliquiae, The Untold), Lustmord ([OTHER], The Dark Places of the Earth, The Word as Power), North Atlantic Drift (Canvas, Monuments, Resolven), Ulf Soderberg (Nattljus, Tidvatten, Vindarnas Hus), and Desiderii Marginis (Deadbeat, That Which is Tragic and Timeless, Seven Sorrows).
Also, some good soundtracks I've found work well are:
From Hell (Trevor Jones), Nosferatu (John Zorn), Hellsing Original Soundtrack - Raid (Yasushi Ishii), The Ring / The Ring Two (Hans Zimmer, Henning Lohner, Martin Tillman).
Any more suggestions are welcome.