So I downloaded few videos from Youtube, but some of the audio is muted due to copyright issue.
Using iMovie:
1. Import video and audio you going to use into iMovie. You can use the button 'Import' or just drag
2. Select entire clip & drag the video to timeline.
3. Select the video and goto menu Modify > Detach audio
4. Select entire clip of your intended audio & drag to timeline
5. Trim & sync accordingly
6. Share and done =)
