How to add a MP3 music background to a video ?
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As the video source was filmed on a smartphone change the screen size
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Upload the MP4 video
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Upload the MP3 music file
Your video will start automatically to record 
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Let the video play until the end and Download your video
Here is the video with a MP3 background music made with 011.video
Audio source : https://www.youtube.com/watch?v=Zcwp5v5KPrw
Video source : https://www.youtube.com/watch?v=n6ZGa2ofjSs