O Brother Where Art Thou?
Carousel (sorry, but I love the music - and yes I know it is not a music themed movie).
Call Me Madam - on the same basis, but also because I love Ethel Merman & Donald O'Connor acting & singing together. A great dynamic.

Latcho Drom (brilliant! Follows the paths the gypsies have taken through different countries and shows their music in each place and you can see how it has picked up local influences but kept the original foundations)
And my vote for Black Orpheus, too - although I haven't seen it for a decade or so.

Electric Dreams. And this isn't a music-themed movie either, but it has a poignant musician's scene in it where the female lead, a cellist, gets into a lift and is jostled just as the doors close and the neck of her cello is broken completely. The others in the lift, and also the male lead/love interest don't get why she is so devastated. I get it. Mudcatters would get it.

Helen