Instructions how to add sound to a video, how to change the sound of a video file in Ubuntu.
Got to the "Ubuntu Software Center", search for "Avidemux" and install it. sudo apt-get install avidemux
Start Avidemux avidemux Open the video file.
Select Audio > Main Track Select Audio Source: "External MP3" "Browse" for the desired "External File" and confirm with "OK"
While saving the coding will be done, i.e. the sound of the selected audio file will be added to the video.