Howard had only a rudimentary understanding of the violin. During closeups, he kept his hands by his side as violinists bowed and fingered the board. This same trick was performed by Isaac Stern and John Garfield a few years later in Humoresque (1946). (Ingrid Bergman did have some knowledge of piano, and her fingering is correct.) The actual music was dubbed for both: Toscha Seidel for Howard and Norma Drury-Boleslawski for Bergman.
When Selznick fired the cinematographer Harry Stradling Sr. and hired the great Gregg Toland to take over the photography of Selznick's remake of the 1936 Swedish version of "Intermezzo", he asked Toland how it was possible that Bergman looked so beautiful in the original European production and so ghastly in his Hollywood version. Toland replied, "In Sweden they don't make her wear all that makeup." Selznick immediately ordered retakes with the "natural look" which so dazzled the world a year later when he loaned her out to Warner Bros. for their production "Casablanca".
It is interesting to note that almost all the musical content of the film is derivative of Norwegian composers, notably Grieg and Sinding (the composer of "Rustle of Spring."). This is rather curious considering the central characters are Swedish and much of the setting is in Sweden.