Hack a video soundtrack and use it as music background for another video

How to add a music background into a video from another MP4 audio soundtrack  ?

1. Upload an image    then move it then click once to freeze the position. Circle the image to rotate it
2. Upload a GIF image    then move it then click once to freeze the position
3. Upload a video    with a soundtrack from your device
4. Move the image outside the canvas by the left (this is an easy hack isn’t it)
5. Start to record    with the audio track from the MP4 video hidden
6. Let the music play until the end then
7. pause  your video
8. download your video

Here is “Video sample with the Deliverance movie soundtrack as an audio background”