Stewart has lent his voice to a number of projects. He has narrated recordings of Prokofiev's Peter and the Wolf, Vivaldi's The Four Seasons, C. S. Lewis's The Last Battle (conclusion of the series The Chronicles of Narnia), Rick Wakeman's Return to the Centre of the Earth, and as the narrator in The Nightmare Before Christmas, as well as numerous TV programs such as High Spirits with Shirley Ghostman. Stewart provided the narration for Nine Worlds, an astronomical tour of the solar system. He is also heard as the voice of the Magic Mirror in Disneyland's live show, Snow White - An Enchanting Musical.

He also was a voice actor on several animated films, including The Prince of Egypt, Jimmy Neutron: Boy Genius, Chicken Little, The Pagemaster, as well as the English dubbings of the Japanese anime films Nausicaä of the Valley of the Wind by Hayao Miyazaki and Steamboy. He voiced the pig Napoleon in a TV adaptation of George Orwell's Animal Farm and guest starred in the Simpsons