Meet with the actor at the ADR stage to record a new voice track. The actor must be prepared to match their mouth movements exactly and recreate the feelings and acting of the original production. This can be a challenge to both long time professionals as well as beginners, since they are standing in front of a microphone in a studio, or soundproof room, acting alone. It will take a professional to match the acting and movements of their mouth so to make it realistic and as they were when it was being filmed.
Record the new track. Because image and sound are recorded on two different mediums altogether (in film) and put together later in the production stage, you can rerecord sound and have the actors lip sync. Dubbing the sound track on film will show the picture from the film shot on screen to the actors, but rerecording the dubbed voices is on the separate sound track. This is why there is both a Director of Photography and a Sound Recorder on a traditional film.
Redo the sound on video or digital by overriding the sound portion and recording new audio on the same video while leaving the original image intact. In a video post-production facility, professionals can separate the images and sound on video into tracks and just rerecord the sound. Digital Mixers, the new personnel in production, are needed for audio overdubbing and any sound tweaking needed. Image and sound are mated together but, again, you can fix, play with and redo just about anything involving sound with digital technology.