I finished BELLS ARE RINGING this afternoon (all the extras, too; I had never seen Dean Martin's cut number. One of Jule Styne's less memorable songs).
Dean Martin was perfect casting as Jeff Moss, but honestly, is there a worse example of lip-syncing by a major star than Dean's in "I Met a Girl"? I mean, there are times when we hear his voice on the soundtrack, and he's not even moving his lips. Often he comes in after the voice has begun singing, and it's clear he's not sure of the words either. Just terrible.
Susan Hayward's "I'll Plant My Own Tree" isn't great either, but since Eileen Wilson sang the song on the soundtrack album but Margaret Whiting dubbed her for the actual movie, I wonder which track Susan was lip-synching to. That might be why her miming isn't very good. She's much better lip-synching in other films, whether it's her voice or someone else's.