... and adding to my earlier response - I just watched the DVD specials; there is an interview with Williams, who states that Kubrick specifically had requested Spielberg to leave the "Rosenkavalier"-waltz in the film, but did not say why or where. So Williams included it "as an homage to Kubrick" in the Rouge City-scene, mentioning that it was difficult to find a place where it would fit, "without fully realizing what the connection in his (Kubrick's) mind was".
Have fun!