krisbkk says in the accompanying notes at YouTube:
Most of the song in the film is Monroe's own voice but she needed help in two phrases -- "These rocks don't lose their shape, diamonds are a girl's best friend", and at the beginning with a series of high-pitched "no's", all of which were dubbed in by the soprano Marni Nixon.Thanks to Doug.
The song was listed as the 12th most important movie song of all time by the American Film Institute.
Monroe's rendition of the song has been considered an iconic performance and has since been copied by other entertainers ranging from Madonna and Kylie Minogue to Geri Halliwell, Anna Nicole Smith and Thalia.
