You pull it up in your video editor (natch!) and then use your mic to record your voiceover as it plays. Then in the video editor drop in the audio track you needed where you want.
This is not an offer to detail how to use or select a video editor.
I have a few FLV/AVI files that have no voice overs or sound. I want to put some MP3 files on them. How do I go about it?
Appreciate your response.