I always assumed the opposite because to my limited listening skills, the sound of that singing voice matched up well in comparison to Lancaster's speaking voice. Another actor with a good singing voice was Robert Mitchum.
I gave the video presentation a grade of 4.0 on a scale of 1-5 with 5 being the highest grade. As to Lancaster's performance, it's my opinion that he gave one of the best acting performances ever captured on film. I think it's up there with Mitchum/The Night of the Hunter and Scott/Patton...