Dubbed singing
"Down to Earth" (1947) may be one of the few film musicals in which literally every cast member's singing voice is dubbed.
It's common knowledge that Rita Hayworth did not do her own singing on film (with the possible and disputed exception of the "slow" version of "Put the Blame on Mame" in "Gilda"); she's dubbed by Anita Ellis here. But Larry Parks, Marc Platt and Adele Jergens are all also dubbed as well. Even the chorus members are dubbed; the three beauties who appear at the beginning of the "Muses" number (I think the middle dancer may be Carol Haney, who was with Jack Cole's dance troupe at the time) are obviously post-dubbed.
Incidentally, Adele Jergen's singing voice is supplied by the very distinctive Kay Starr, who was a band singer and then a top popular songstress in the 1950s.