That cartoon would be- Plane Crazy. Ub had animated about 2/3rds of the film before Walt Disney came upon the magic of sound. Since Plane Crazy had been envisioned as a silent cartoon, he chose to scrap everything and come up with an idea for a cartoon that would feature sound. That cartoon was Steamboat Willie.
Steamboat Willie was a sensation, catapulting Mickey Mouse to superstardom. Despite the fact that Walt's brother Roy was against it, Walt was proven right when Steamboat Willie broke every record. Afterwards, Plane Crazy was reworked with sound and Mickey Mouse became a worldwide celebrity.