One of Disney's four "Package Films". During the Second World War, the studio lost a lot of manpower and resources which left them with countless unfinished ideas too long for shorts, too short for features. So, inventive as they are, they stuck short ideas together into feature length movies. The other three were The Adventures of Ichabod and Mr. Toad (1949), Melody Time (1948) and Fun & Fancy Free (1947).
In "The Whale Who Wanted to Sing at the Met", all of the operatic vocals - from the bass up through soprano, and even the chorus - were sung by Nelson Eddy. By tinkering with his home recording equipment (his father was a machinist), Eddy was able to sing and record sextets with himself for the soundtrack.