Add the soundtrack to the movie
You can export a copy of the movie file containing the soundtrack you create in GarageBand. Your soundtrack (which includes the movie’s audio, unless you mute the track containing it) replaces the existing audio in the movie.
Export the movie with the project audio
Choose File > Movie > Export Audio to Movie.
Choose the export quality from the Preset pop-up menu.
Explanatory text for the chosen preset appears below the Preset pop-up menu.
Enter a name and location for the movie, then click Save.
The exported movie contains a mixdown of the project audio. If the project audio extends past the end of the video, the exported project is lengthened to match the length of the project audio.