Before final animation begins, a preliminary soundtrack or "scratch track" is recorded, so that the animation may be more precisely synchronized to the soundtrack. It is common to synch animation to a pre-existing soundtrack.
The scratch track used for animation usually contains just the voices and songs the characters must sing, and preliminary music tracks.
Sometimes the soundtrack may be synchronized to a pre-existing animation. This is called dubbing and if it is a voice track it is called a voice over. This also may be done for parts of final editing in order to make corrections and modifications to an almost completed animation.
Each track is usually done on a different layer to allow for individual modifications on one track without disturbing another track. There may be a separate track for voices and another for music, and another for sound effects. Multiple versions of each type of track may be created depending on the complexity of the production.