According to IMDb, this is a silent cartoon though I noticed various sound effects and a sort of voice for some of the characters. I assume that like many cartoons from the late 20s that sound was added later--a very common thing once "Steamboat Willie" made a name for Disney. In fact, Disney himself went to his two previous Mickey Mouse cartoons and added sound to them ("Plane Crazy" and "Galloping Gauchos"). So, don't be surprised if you, too, notice that this is a partially sound cartoon.
I've seen several other early Felix cartoons and have found that they were among the very best of the time. Much of this is because the character so often the 'fourth wall'--stepping out of the cartoon so to speak. While this occurs in other films, the Felix ones do it almost constantly--making them very goofy fun. For example, Felix sees the musical notes as characters sing and he grabs them and uses them in various funny ways.
The plot involves Felix sneaking between Romeo and Juliet. Felix is determined to win the fair Juliet himself and battles his to do it. As for Juliet, she seems pretty content either way! Overall, a lot of silly and imaginative fun.