You can use Apple Motion to create motion graphics that react to audio. In this example I'll show you how to have a circle scale to the beat, a visual equalizer effect, and a waveform effect. You can take these and export them to use in iMovie and other video editors.
Apple's Motion app lets you create all sorts of video special effects. In this tutorial, we'll take a graphic logo and make it explode into 10,000 particles in 3D. The particles will fit the shape and color of the image. Then we'll look at how to reverse it so that the particles come out of nowhere to reform the graphic.