Motion graphics, which bring logos, text, and other illustrations to life, are greatly enhanced with sound effects. Sound design for motion graphics presents unique challenges due to their short, abstract nature. Sound designers must identify opportunities for adding sound, decide on the sound's character to match the visual graphics, synchronize sounds with events, and align sonic properties with motions. We introduce MoSound, an interactive system that helps with all steps of this creation process. We designed the interface of MoSound based on formative studies with practitioners and implemented the system as a combination of visual event detection, spatial attribute mapping, and generative sound stylization. We demonstrate MoSound on a variety of examples, showing that it is capable of creating high quality soundtracks while being accessible to novices.