Make a directory for your video project and sub directories for both the sound you remove and the sound you will eventually create. Planning on how to organize and keep track of your files as you work on your project is essential to keep things from getting lost and to keep track of prior versions should you need to go back to something.
Open your movie program. In either iMovie or Windows Movie Maker you will be asked to name the project file and assign it to a directory. Make sure, before you click 'OK' in either program, that the directory you created in step 1 is reflected in the path for your project.
Import the video that you want to work on by selecting "File" and then "Import" and selecting your video from your files.
Click on the icon in the lower left control bar of either program that will change your project viewer to the "View Clip" mode.
Click and drag your clip into the project viewer and release the mouse button dropping it in.
Right click on the video. This will bring up a submenu that will offer your the choice of "Extract Audio." Select this. The program will automatically remove the music from the video and place it into the audio track below.
To completely remove the music from the video, click on the audio track and select "Delete" or press the "Del" key on your keyboard.