So why you need a software installation. On Mac, there is iMovie can do this easily, or Final Cut Pro (the one I use, I did this with example: http://www.youtube. com/watch? v = sSK9uHEFjdg), which is a professional editing software, used by all the pros. But already we must be on Macs, which is quite rare. There must be the same on pc, but I do not know.
You start by importing the movie into the machine. You record your voice or that of another and you incorporate it into your movie. You exported the following result that you want to do a dvd or a movie for the internet, etc.. It sounds easy like that, but it is a series of treatments that are complex to learn and master. If you tell me exactly what you want to do I can help. But I must know exactly, because there are so many possibilities !