Depends on the type of video. The audio has to be demuxed from the video then edited in the likes of Audacity. The doctored audio is then muxed back with the video. For avi, I would demux/mux using AviDemux. For mp4, YAMB. DVD movie requires demuxing with PgcDemux and remuxed using MuxMan. As long as it doesn't involve multiple audio streams or DVD's with menus,it's straightforward.