You know, it's sad, but I made a little list of everything in my iTunes that worked for my armies. There's a fair bit of overlap, but a lot of it works for several armies in the game.
Lions Rampant Space Marines:
-Audi Famam Illius (Theme to Super Smash Bros Brawl)
-Ascenndit Deus (Gregorian Monks)
-Benedictus (Gregorian Monks)
-Brothers in Arms (Gregorian Masters of Chant-- a group that sings pop songs like monks, but aren't monks)
-Dirge for November (Opeth)
-Elysium (John Williams)
-Fields of Hope (Gregorian)
-Hallelujah (Gregorian)
-Hero (Dave Matthews)
-Holding Out For a Hero (Bonnie Tyler. Yes, it's a lame 80's song, but the line "Isn't there a white knight upon a firebrand steed?" Inspired the white armor of the Lions Rampant, and originally gave them the Chapter name White Knights before I realized that wasn't a safe name around here...)
-Knockin' on Heaven's Door (Bob Dylan)
-Organ Voluntary (Gregorian)
-Put Your Lights On (Santana)
-Save a Prayer (Gregorian Masters of Chant)
-Stairway to Heaven (Gregorian Masters of Chant recording)
-Strength and Honor (John Williams)
-The Chosen Ones (Dream Evil)
-The Resurrection (Gregorian)
-The Saints are Coming (U2 and Green Day radio edit)
Adepta Sororitas Order of the Bloody Rose:
-Amazing Grace (Faith Hill recording)
-Ave Maria (Sarah Brightman recording)
-Ave Mundi Spes Maria (Gregorian)
-Down to the River (O Brother Where Art Thou Soundtrack-- what a horrible movie, though...)
-Feels Like Fire (Santana featuring Dido)
-Hallelujah (Gregorian)
-Into the Fire (Thirteen Senses)
-Organ Voluntary (Gregorian)
-Salvation (The Cranberries)
-To the Death of Sin (Nobuo Uematsu)
-The Resurrection (Gregorian)
-The Saints are Coming (U2 and Green Day radio edit)
-Where the Wild Roses Grow (Kylie Minogue & Nick Cave)
Sons of Horus:
-Back in Black (ACDC)
-Bleak (Opeth)
-Hurt (Johnny Cash)
-I Fought the Angels (The Delgados)
-I Fought the Law (And I Won) (Sex Pistols mix)
-Ring of Fire (Johnny Cash)
-Paint it Black (The Rolling Stones)
-Sympathy for the Devil (The Rolling Stones)
-Straight to Hell (Glisli)
-The Chosen Ones (Dream Evil)
-The Four Horsemen (Gregorian Masters of Chant)
-Under the Weeping Moon (Opeth)
Thousand Sons-
-Black Magic Woman (Santana)
-Crumbling of Time (Nobuo Uematsu)
-Devil (Stereophonics)
-The Sound of Silence (Simon and Garfunkel)