I just watched "Jumbo" on DVD and it seemed to me that Stephen Boyd's singing voice sounded a lot like Vic Damone. Whoever provided the vocalizing did an admirable job, though the voice sounded nothing like Mr. Boyd's speaking voice. I checked IMDb to see who actually did the dubbing but there was no information there. Did anyone else hear Vic Damone?
PS - I agree with the previous poster that John Astin played the hapless airplane pilot. Maybe MGM didn't credit him because the airplane hadn't been invented yet.