GEORGE GERSHWIN: SONGS. Marni Nixon, soprano; Lincoln Mayorga, piano. Reference Recordings RR-19. This recording of Gershwin favorites is polished and professional, yet also sparse and dry. One could not ask for better intonation by the singer, or better balance by the pianist. Yet the performances have a thin, almost lonely, sound. Nixon's voice is not rich in its lower registers. Songs like "Embraceable You" ask for more darkness in the voice; and nearly all the songs, from "I Got Rhythm" to "Of Thee I Sing," ask for more stretching and surging of tempo than the performers allow.