Don Knotts's (T.W. Turtle) brief singing role is filled by session singer Rick Logan.
For the singing role, Ashley Peldon (Darla Dimple) was replaced by Lindsay Ridgeway.
For Sawyer, the non-singing voice was provided by Jasmine Guy. Singing, Natalie Cole takes over.
The Other Marty: According to Lauren Faust, Jasmine Guy was a last-minute replacement for another actress, possibly Nancy Giles. She wasn't hired until all of Sawyer's animation had already been completed!
This would explain why Sawyer's speaking voice and singing voice sound radically different in the final film. Giles' voice would have been a better match for Natalie Cole.
Screwed by the Studio: Just before the film came out, Time-Warner purchased Turner and put it on a crowded Easter holiday weekend while giving it next to no marketing whatsoever.