"Mooooooooooooo."

Milky White doesn't do much, but that doesn't keep her from being Jack's best fried. Jack isn't very demanding you see. She goes through a dry patch, milk wise, and becomes a source of revenue in another way. Milky White is a very important character to the story, if not a very vocal one.

Musically:

Cows can't sing.

Playing Milky White:

In the original production, Milky White was a prop. While this provided some good comic moments, it has become prefereable to have an actor in a cow suit perform the role. Either is quite acceptable, as long as she's as white as milk.