How to add audio to a video file

Use AVIDemux. It's really quick and simple, and doesn't require reencoding of the video file so it's super fast! If you can't get that working, the free trial of AVS Video Editor 8.1 works fine. The only problem is a very small watermark in the center of the video which disappears after 60 seconds. Using this software does give you the benefit of more editing features like audio, text and transitions effects.