Merging and synchronizing video clips for musical harmony

Hi, and welcome to the Apple (user-to-user) Community.

Re: "Merging and synchronizing video clips for musical harmony"

Wondering whether you are using an iPhone, iPad, or Mac, for iMovie.

If you are not already using it, the iMovie User-Guide may be helpful in determining what is actually possible to achieve with the app.

You can click "Table of Contents" (at the top of the welcome page that opens from the link), to see various topics leading to detailed "how to" articles

iMovie User Guide for Mac – Apple Support

iMovie User Guide for iPhone – Apple Support

iMovie User Guide for iPad – Apple Support

iMovie Support website:

iMovie Support

Get help viewing, editing, and sharing movies on your Mac, iPhone, iPod touch, and iPad.

and

There's a "Get Support" link at the bottom of their webpage.

iMovie - Official Apple Support

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Re: "so a harmony clip can correctly overlap a melody clip".

An audio app (where you can clearly see the waveforms, and can add audio tracks), like Garageband, (a free Apple app), may be a great help in synchronising the harmony and melody if the audio files and video-image files are separate, as they may be when exported to Apple devices from some video cameras.

GarageBand User Guide for iPhone – Apple Support

GarageBand User Guide for iPad - Apple Support

macOS app user guides: GarageBand for MacUser Guide - Apple Support

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There are third-party (non-Apple) apps available online that may help sync audio and video: Have not tried any of these myself, so do not know how reliable they may be: An example: https://www.canva.com/features/beat-sync/

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Adobe's "Premiere" Pro app was often the video-editing app chosen by professionals, for perfectly aligning and synchronising both audio and video. App now seems to be one of the apps only available by monthly subscription.

All the best :-)