How do you add MP3 sound effects to a video ?
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First upload your video
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Open a new browser tab and get a noise from an online sound database (For instance : https://freesound.org). You can also upload your own MP3 sound inside the tab browser with the command CTRL+O.
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Come back to 011.video web app and start the capture
Don’t forget to Share the audio. -
Start to play your sound. If the audio length is shorter than the video length select the audio loop.
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Go back to 011.video and start recording
For your ears comfort, mute browser tab to get only one audio source send to your speaker. -
When it is finish pause
and download 
your video with a background sound.