If you need to add some sound to your videos and you are using Linux, there is a very easy way to add it.
You can for example add your favourite song into your video, or even record your voice and put it in your video.
mencoder source.avi -o destination.avi -ovc copy -oac copy -audiofile /path_to_the_mp3_file/file.mp3
By telling mencoder not to compress the source video nor the audio the hole procedure will take a few seconds.
Enjoy it !!!