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Removing Audio from Video
You can open your AVI e.g by virtualdubmod and make selection for video "direct stream copy". You can disable unwanted audio in Stream/Stream_list by clicking "disable" button for unwanted one. Then you can save that with another name.
If you are having more problems take a look at the thread I posted it might help.
After you remove the audio just open the video again and add your music in there. You earlier said you knew how to do that so I won't go into that part.