Fantasia was one of Walt Disney's most ambitious works. Made in 1940, it involved animation segments that were built around several famous classical works. The movie required very specific projectors to cater to the vivid color of the animation; it was also the very first film to ever be recorded for surround sound. The specific demands for the film were so high that most conventional theaters at the time refused to screen it, so, Walt came up with the solution of showing the film via roadshow tours rather than a wide release.
The original roadshow version was over 125 minutes long and contained a few differences from the commonly-seen 115 minute version. Classical enthusiast, teacher, and critic, Deems Taylor did the inter-song monologues. In the roadshow version they were much longer and more detailed, though the monologues were cut in future releases of the film to make it more "accessible".
Then, in 2000, Disney decided to restore Fantasia to its original roadshow version. Taylor's original extended audio tracks had deteriorated so greatly that it was virtually irretrievable. Corey Burton had to be brought in to do the complete dub over again for consistency's sake. Taylor's voice clips have never surfaced, though Disney has reportedly been putting painstaking effort in to restoring the film to its complete original version.
Other "missing" elements of the film are a few offensive scenes of a black centaur (named Sunflower) serving a white centaur, however, these scenes are widely available on YouTube, despite numerous claims of the footage being lost. Disney, understandably, did not restore said scenes and show no plans to put them into any future releases.