why don't you try to mix your audio files using audacity or sony acid xpress or any one audio editors you may familiar
here some of freeware to checkhttp://www.techradar.com/news/software/applications/best-free-editing-software-15-on-test-1058619?artc_pg=2
after mixing all the tracks you can insert them with the video for final touch..
some of free video editors...http://www.akascope.com/2011/07/15/free-video-editing-software/
i used some audio & video editore a long time age.. not sure it can be done like i mentioned here..
sorry if its wrong