Pot o'Gold

1941 musical starring Jimmy Stewart and Paulette Goddard. From IMDb:
Jimmy, the owner of a failed music shop, goes to work with his uncle, the owner of a food factory. Before he gets there, he befriends an Irish family who happens to be his uncle's worst enemy because of their love for music and in-house band who constantly practices. Soon, Jimmy finds himself trying to help the band by getting them gigs and trying to reconcile the family with his uncle, an avid music-hater, all while winning the heart of the beautiful Molly! (via On the Other Foot)