I don't think there is a built in function and I've never really looks at much of the Audio and Video portions of MV but I assume it's connected to Pixi somehow. I don't believe the videos are controlled through the game objects but through the core.js. Graphics to be precise. You could try Graphics._video.stop() but I don't see anything related to a stop method. Maybe someone who has dived into the audio and video of mv can help you out.