When Varese released the soundtrack of Alfred Newman's score for The Snake Pit ('48), Clayton's vocal of "Coming Home" was not on the CD. I think I was told that it didn't exist. The same was true of the vocals of Spencer Tracy, Charley Grapewin, John Carradine, and Freddie Bartholomew when Intrada released the complete soundtrack of Franz Waxman's score for Captains Courageous ('37).
However, when FSM released the soundtrack of Herbert Stothart's score for Random Harvest ('42), Greer Garson's vocal of "She Is Ma Daisy" was indeed on the CD. Does that mean that Garson was lip-syncing to her previously recorded vocal, but the others were singing live on their sets?