The term 'ADD SOUND EFFECTS TO' can be summarized by the crossword solution DUB, which is a three-letter word. Dubbing is the process of adding sound effects to a film or video, such as dialogue, music, and sound effects. It is usually done in post-production, after the filming has been completed. Dubbing is an important part of the filmmaking process, as it helps to create a more immersive experience for viewers.