Sauron -
Welcome to the Black Parade by My Chemical Romance
Elwing -
I Believe I Can Fly
Melkor to the Valar -
Be Prepared in the Lion King
Cirdan -
Row, Row, Row Your Boat
Feanor and his Noldorin Friends -
The Kinslayer by Nightwish
I was watching Star Trek: The Undiscovered Country just before I saw this thread then I remembered Kirk and McCoy singing Row Row Row your Boat to Spock.