Rachmaninoff didn't write any pieces of the sort. The closest I can come up with is a Romance for Flute, Cello, and piano which he wrote in 1890 but didn't coutn with an opus number. Perhaps the singer is singing the flute part? There're also two other un-numbered Romances from 1890 - a piece for cello and piano, and a piece for violin and piano.I don't know the works themselves, so I'm afraid I can't help you with the melodies. But that's the closest he comes to Bass, Piano, and Voice.