Intellectual Property and ownership of work created in game worlds

Once again, I am thinking about who owns a "work" that is created in a game world. Yes, at this point I believe it is pretty clear that the game provider owns it. When it is something more than a silly short video, who will own it? If I write a song in a game world using the game worlds mail system, is the song mine or the game providers?

Anyway, here is another short that is pretty funny. I think this one was created using HALO2:
link

Leslie