best free program to add sound or music to our video,s

I use VSDC Free Video Editor.  I can't say whether it's the best.  

Eric Matyas http://soundimage.org/

Near all Video editors will add music ... I use Windows Movia Maker which also has direct link to Free Music Archive or to any music you have on your PC. It can be overlay, underlay or completely replace any audio track you may already have.

Note that online video hosting sites such as Youtube can and do often mute soundtracks if a Copyright complaint received.

Nigel