The actors mentioned above, but the film also features some famous musicians as the characters' singing voices: Andrea Corr of the Corrs as Kayley, Bryan White as Garret, Steve Perry as King Arthur, and Celine Dion as Lady Juliana.
There is also Sir John Gielgud as Merlin...who gets all of two lines. Why did they even bother? Granted, the story was more about Kayley and if he had gotten involved it would have been a major Curb-Stomp Battle...but at the same time one wonders why Merlin did absolutely nothing to help other than sending Ayden to find Excalibur (and, it's implied, to be Garret's friend and helper in the forest). Especially if "witches" were involved in giving Ruber his potion. A lot more potential plot which would have actually justified casting such a well-known actor in the role...all gone to waste.
Non-Singing Voice: Most of the cast, possibly because this wasn't initially written as a musical, so whether a given performer could sing would not have been a concern at the time he/she was cast. In addition, some of the original actors can sing, but it's possible that the songs were added in so late that either the contracts were already set in stone or the actors were no longer available.
Old Shame: According to Lauren Faust, all the animators (herself included) disliked working on this movie.
What Could Have Been: The film was originally intended to be darker, edgier and generally a more serious adventure film before being retooled into a more family friendly Disney style film.