Interesting to note that in the earlier Warner's musical "Yankee Doodle Dandy", that Bayes is portrayed as a "heavy", taking Mary Cohan's song away from her.
Insofar as voice dubbing is concerned, all actors are ultimately dubbed, whether with their own singing voice or someone else's. Nobody sings "live" on camera. Not much different than today's music videos where the performer is only doing a lip-sync to the recording (and you thought they had all those electronic instruments and back-up singers hidden in the bushes, right?).
This film is still enjoyable for the music and the performances of the actors. I think it's well worth the time to view it.