Murphy may have died of natural causes

Her on-screen roles declined in recent years, but Murphy's voice gave life to numerous animated characters, including Luanne Platter on more than 200 episodes of Fox's "King of the Hill" and Gloria the penguin in the 2006 feature "Happy Feet."

She is due to appear in Sylvester Stallone's upcoming film, "The Expendables," set for release next year.

Murphy credited her mother, Sharon, with being a key to her success.

"When I asked my mom to move to California, she sold everything and moved out here for me," Murphy said. "I was really grateful to have grown up in an environment that was conducive to creating and didn't stifle any of that. She always believed in me."

She dated Ashton Kutcher, who costarred with Murphy in 2003's romantic comedy "Just Married."

Kutcher sent a message on Twitter Sunday morning about Murphy's death: "2day the world lost a little piece of sunshine," Kutcher wrote. "My deepest condolences go out 2 Brittany's family, her husband, & her amazing mother Sharon."