The music performed in "Symphony Hour" from 1942 is actually the Overture of Franz von Suppé's operetta: "Leichte Kavallerie (Light Cavalry)". This piece was referenced in the scores of numerous other Disney Cartoons, including the 1937 Mickey Mouse Cartoon "The Worm Turns (1937)", the 1945 Donald Duck Cartoon "Cured Duck (1945)" and the 1949 Goofy Cartoon "Goofy Gymnastics (1949)".
"Symphony Hour" from 1942 would be the second to last animated short in the Mickey Mouse Cartoon series until the end of World War II. The last Mickey Mouse Cartoon is "Pluto and the Armadillo (1943)" from 1943 and the first released after the war is the 1946 Mickey Mouse Cartoon "Squatter's Rights (1946)".
Several characters appear in both this short and the earlier short 'The Band Concert.' Interestingly, none of them play the same instruments in both cartoons (Mickey led the band in both films, however).
The piece being played is the "Light Cavalry Overture" by Franz von Suppé.
This is the only Mickey Mouse cartoon to feature Mickey Mouse actually threatening to shoot Donald Duck with a handgun. The scene was removed from television showings in 2000.