All throughout Stuart Little 2, Eleanor is trying to get Martha to say words, but she doesn't comply. Martha is, of course, a baby, so her personality is such as clingy to her mother, mess maker, and curious of her brothers George and Stuart.
At the end of the film, when Margalo leaves to fly south for the winter, Martha finally says, "Bye-bye, birdie!" and her family is happy for her.
In Stuart Little: The Animated Series, she is 3 years old and can speak some.