Soooo story time for a little known part of animation history.
Max Fleischer, a jew born in Austrio-Hungary, was Walt Disney's rival on the east coast. He created notable characters such as Popeye and Betty Boop. He also created the WW1 army training films on subjects that included Contour Map Reading, Operating the Stokes Mortar, Firing the Lewis Machine Gun, and Submarine Mine Laying. I don't think I really need to run down Walt Disney's repertoire.
All this to say, Max Fleischer was the first to come up with sound and animation being married together. I know, most of you are thinking that Disney's Steamboat Willy was the first to accomplish that. Nope. Max Fleischer did it four years before Disney with a series of shorts called Song Car-Tune. They were bouncing ball sing along cartoons...he invented those by the way. The reason you think Disney did it first is because Disney actively discouraged the media from mentioning Max Fleischer's work. Disney claimed the credit for being the first to use those techniques. And that was the story that stuck. You know how it all ends from here: Disney is still going and Fleischer's studios are no more.
I will give you a source. But there are better ones out there. None of this is secret, it just isn't talked about much.
Edit: Words for clarification.