It's important to note that while the film itself received several nominations, including Best Picture and Best Director, Marni Nixon was actually the singing voice for Miyoshi Umeki, the actress who played the lead role of Hana-ogi in the film. Nixon's voice was dubbed over Umeki's throughout the film, but she did not receive a Best Actress nomination.
So, while Sayonara won an Oscar for sound recording, it was Marni Nixon who received the award for her work on the film.