Congenital amusia and problems singing a familiar song
As you may know, "congenital amusia is a neurogenetic disorder of pitch perception that may also compromise pitch production." This paper looks at how this relates to signing songs, new or familiar, with or without words. Among the findings: "The current study demonstrates that deviating from target duration and relative time matching patterns as well as making many time errors is more often than not the case in congenital amusia regardless of whether the content to be imitated is more or less familiar." See Song Imitation in Congenital Amusia | Music Perception | University of California Press (full paper accessible here ).