Citizen Kane

In Citizen Kane, music is mainly used to advance transitions and make montages flow. In the beginning of the film, a somber theme using low, quiet brass and strings as the camera slowly moves from the outside of the Xanadu fence at a low angle to the inside of Kane's room as he says his final, famous word: "Rosebud." This theme gives Kane's estate a funereal air and also surrounds Kane's death with a certain mystery because the visuals show that he is very alone at the end. This relatively quiet music sets up a great contrast with the boisterous trumpets and drums that briefly introduce "News on the March" which uses a histrionic narrator to quickly highlight the highlights of Kane's public life. During "News on the March" this narrator is accompanied by a full orchestra and brief videos of Kane meeting important leaders, acquiring wealth, etc.
Later, 5 separate short scenes compose a montage of Kane and his first wife. Their relationship in the first is one of love and gentle nagging toward Kane to spend more time at home, and he's jocund and somewhat willing. At this point, the music involves mainly strings and is soft, nearly romantic. The next is mostly woodwind and strings, somewhat gentle yet as Mrs. Kane is still gently prodding Kane to spend less time at the office. The music is sinking woodwind and some brass as she sadly tells Kane he shouldn't directly attack the president so brazenly. The penultimate is somewhat lower brass and Kane directly orders Mrs. Kane to continue to allow the coarse Mr. Bernstein in the nursery with their son. In the final scene, the strings reappear much sadder as Mrs. Kane dejectedly scolds Kane for his dishonest paper, and Kane vehemently tells her "People will think what I tell them to think!"
Some less noticeable music, usually underscoring, is used in the flashback scenes, and in general the film uses its score as a means of highlighting and characterizing segues.
It is also worth noting that imdb.com refers to six composers, including Max Steiner, as sources for stock music used throughout Citizen Kane.