The lyrics to 'Charging Fort Wagner' open up with part of the Dies Irae. The Dies Irae is a fixed poem used in many Catholic ceremonies and can be easily found online. "Quando judex est venturus, cuncta stricte discusurus," meaning "when the judge is come, he will judge all strictly," and that's a good basis for the beginning of any internet search. That'll allow you to follow the first couple minutes of the 'Charging Fort Wagner' piece.

I haven't had any luck with the rest of the lyrics. A pity they've not been published anywhere.