This is really interesting -- our friend Tommy Tallarico of Video Games Live recently did a show in Austin, Texas, and during the performance, they actually brought former Blizzard composer Jason Hayes up on the big screen via Skype and chatted with him about the Warcraft music they had just played. Hayes worked on some of the cinematic music for Warcraft III, and though he is no longer with Blizzard, he put together some of the new music you're hearing now in Wrath of the Lich King as well.
Hayes talks about how he works closely with the cinematic team to try and come up with an appropriate mood for what's happening on screen as the music plays. Even with early animatics, he says, you can get a good sense of where the scene is going and what the story is about. He also says that the Latin lyrics in his songs aren't necessarily Latin -- he uses a translator online and picks and chooses what sounds best. So not exactly historically accurate, but it's Warcraft -- as long as it sounds good, they can justify it however they like. It's definitely a fun little chat with one of the people behind some of the sweet sounds of Azeroth.