Add MP3 sound to video online

How to Add MP3 sound effect to a video in real time online  ?

1. Upload a video  you want to add a MP3 sound effect over   
2. If the video come with an audio track, mute the sound 
3. Upload a MP3 sound file from your device   
4. If you want to upload a sound from URL double-click inside the canvas and paste the MP3 file link
5. To record video + audio together click on    
6. Pause the video   
7. Download the video with a new MP3 audio effect