The only vocal music I would stick in a game like Freespace 2 would be Lisa Gerrard, Dead Can Dance, Stellamara and other artists in that genre. (which I guess you could call neo-classical/darkwave... a lot of the singers in that genre are singing in glossolalia or are singing in Latin and in such a way that draws you into the experience rather than distracting you)
I actually think it could be pretty amazingly well suited to this kind of game actually. Modders you should check this stuff out.
Here's an example of Gerrard:
http://www.youtube.com/watch?v=QiYqYPEXsJUIt fits perfectly with the epicness of space IMO. Would fit well in a scene where the battle is a lost cause and you're getting torn to shreds or as the backdrop to some cataclysmic event playing out on screen

.
And as someone who listened to metal a ton throughout high school and college and still listens to it occasionally, I definitely don't think it fits a game like FS2. Ambient, ambient-leaning electronica, classical and the above kind of music (which I guess you could call neo-classical/darkwave)
From the rock spectrum the only thing that would be appropriate would be stuff like Mogwai though I still think the above genres would be much better.