Fantastic scene from Sun Valley Serenade - Glenn Miller and his orchestra performing 'Chattanooga Choo-Choo,' with vocals by Tex Beneke and the Modernaires, and then a reprise by Dorothy Dandridge and the Nicholas Brothers.

Chattanooga Choo-Choo

I can't link to it because the video is unfortunately no longer on YouTube, but another all-time favorite of mine is the Vienna Boys' Choir performing 'Omnes de Saba Venient' in the Disney film Almost Angels, a little-known gem.