Waitin' on the Robert E Lee!
In full voice here, in what I found to be a surprisingly appealing biopic, is Larry Parks as Al Jolson, in The Jolson Story (1946).
In fact, Jolson recorded all the songs for the movie of his life, which where then dubbed over Parks when required.
The technique works very well.
And Parks does have a fairly good resemblance to Jolson.
Parks was blacklisted in the early 1950s, and never found steady work in the movies again.