Getting just the right look for the leading lady didn't stop Old Hollywood from casting non-singers as leads in big budget musicals. That's where dubbers come in. They're still used today to mask less than great singers on the silver screen. Before vocals were mixed to sweeten a singer, they were swapped out entirely in post production. One of the most prolific dubbers was Martha Mears. She filled in lead vocals for Lucille Ball (The Big Street), Marjorie Reynolds (Holiday Inn), and Hedy Lamarr (My Favorite Spy). More impressive is how varied her vocal range and type can be.
Lost Vocals on YouTube put together a 13 minute video of Martha Mears' dubbing work in Hollywood. She performed over 50 voice over roles in 12 years. It's overwhelming in the best way possible.