The first eight bars of Beethoven's Fifth Symphony. The first 4 notes of Beethoven's 5th Symphony were appropriated by the Allies as a Leit Motif throughout the war. It matched the Morse code signal for "V", for Victory.
The main march. The vocal version is here. The main theme is never played in its entirety until the closing credits; it's only hinted at. The first time is in the officer's club where we first see Richard Burton. Someone is slowly picking it out on the piano.