I'm watching Blackbeard's Ghost, a formulatic but enjoyable Disney film from 1968 that starred one of my favorite actors, Peter Ustinov. Dean Jones was a Disney leading man in this, The Horse In the Gray Flannel Suit, The Love Bug, etc.
I never knew Jones could sing, but giving it a bit more thought I think that probably every actor can sing - they have the vocal training and the necessary lung power.
So here's Dean Jones singing Being Alive from the musical Company, in 1970. He left the production after only two weeks (and after singing in the cast album) apparently because he was going through a painful divorce at the time.
(So, okay, its not science fiction, but he was in a lot of fantasy movies at Disney, so that counts).