Sally in stageCabaret is supposed to be a bad singer (at least in the original, pre-film release, productions). Judi Dench isn't a singer and she was the original Sally in the West End production (1968), so I would think the actress isn't (wasn't?) required to be much of a singer to do the part. But anyway, I think most singers do have a coach, if only to help keep their voices in shape.
Can you imagine expecting to see Gwen Verdon and Chita Rivera and getting Liza Minnelli and Chita Rivera? Still, I bet some people were upset about it.
The last time I saw a production on Broadway