A ways back, we served up a Sinbad, Jr. short that was produced by Sam Singer and Trans-Lux. Well, as we documented, American International wasn't happy with the product, so they turned the animation over to Hanna-Barbera. With that came a casting change, as Dallas McKennon (Daniel Boone), who had previously worked for Singer on Courageous Cat five years earlier, was cut loose in favor of Tim Matheson (Jonny Quest) and Mel Blanc (The Flintstones, Secret Squirrel, etc.).
In "Mad, Mad Movies", Sinbad is roped into making a movie for a desperate director (Blanc, using his Cosmo Spacely voice from The Jetsons), looking for a new star. This was a ream that would be used with other characters at other studios over and over again through the years. Matheson would stick with his mature voice for later roles (Space Ghost, Samson & Goliath) the next two seasons.
Typical of the period.