Heres what I got:
Imperial Armies: Angry music, such as Thrash Metal, or Holy sounding/Classical music
Chaos: Thrash metal or, for some reason, I think that Rapcore fits Chaos well
Tau: I don't know, for Tau, sorry.
Necons: Classical music with a heavy, ponderous feel
Eldar: I am not sure what its called, but there is a very distinct brand of Arabic/Islamic music that revolves around a series of long, low, moans. I think it fits the Eldar well. If you have seen the film Black Hawk Down
you know what I am talking about.
Dark Eldar: For some reason, I think a very fast punk, possibly with a rap overlay, fits the True Kin well. Mindless Self Indulgence I think, would be good.
Tyranids: The sounds of screaming and insectoid screeches are all the music you need.
Orks: Country Music
. I think that the idea of Orks just destroying everything to a country soundtrack is funny.