I have a large collection of music from video games (albeit much of it too distinctive in sound, or too retro to fit), and the general aural aesthetic of FfH is not to my personal taste. So I've been busy.
Amurites:
Welcome to Rapture, from Bioshock. Something about it has a certain magical sound.
Calabim:
Door of Holy Spirits, from Castlevania: Symphony of the Night. That their theme would come from Castlevania was basically written in stone, it was a matter of choosing the right one.
Clan of Embers:
Abandoned Pit, also of SotN. I like the balance between the civilized and the primal for the Clan.
Infernals:
Curse Zone, again of SotN. (Sensing a pattern?) I think it speaks for itself. Do not listen to during dark nights in creepy rooms.
Khazad:
Valley of the Fallen Star, of Final Fantasy VII because... well, it just fits, somehow.
Kuriotates:
Lazy Afternoons, from Kingdom Hearts II. Really, the KH soundtracks mostly mix with Erebus like water and
oil tar demons, but for Cardith Lorda, I figured an exception was in order.
Luchiurp:
Modron Cube, from the Planescape: Torment soundtrack. It's got a good, light-hearted tinkering sound, but still works in the dark world of Erebus.
Malakim:
City of Jerusalem, from Assassin's Creed. It's got a good, religious-leader vibe, and fits with both the feel of Erebus and Varn Gosam in general.
Mercurians:
Lost Painting, again from Symphony of the Night, though they very briefly had
J-E-N-O-V-A from FFVII. I can offer no rational explanation, simply that it seemed to "fit".
Sheaim: They've really bounced around in diplo music. I started with
Those Chosen by Planet, from Final Fantasy VII, but after a while, it struck me as too distinctive a tune for them. I switched them to Planescape: Torment's
Fortress of Regrets for a brief period, but that was even more distinctive, with its heavy use of the Nameless One's leitmotif, so I toyed with the
Fortress of Regret's battle theme instead. But while the general sound of Torment fits, the battle theme proved too aggressive for what is a (very unconventional) builder civ, so they're presently on
Ignus' Theme.
As for the rest: the Hippus, Illians, and Lanun remain untouched. For all others, I can't actually recall the source; typically they were picked up from elsewhere in the FfH mod, these forums, or the Warhammer mod... or I've just plain forgotten, and might edit them in later.