Apple's iMovie is a simple editing program that provides an array of basic effects and features. While it can be very beneficial for new editors, those with more advanced skills typically upgrade to a more sophisticated program such as Final Cut Pro or Adobe Premiere. If you want to loop an audio track in a selected video project, there is a way to do it in iMovie. Due to the program's limitations, however, the process involves manually copying and pasting the track several times.
Open iMovie, then open the project on which you're working.
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Click the "musical notes" icon under the Video Player, on the right side of the iMovie program pane. This will display all the music and audio files in your iTunes folder, as well as iMovie's sound effects. Click the music file you want to use and drag it to the place in the Timeline where you want the audio to start.
Click the audio file in the Timeline, then click "Edit" and "Copy" to make a copy of it.
Move the Timeline cursor to the place you'd like to insert more audio. Click "Edit" then "Paste" to paste your copied audio file in that location. Once you've copied the file, the cursor should be at the end of the piece.
Continue clicking "Edit" then "Paste" to copy more versions of the audio track onto the Timeline. Stop when you've added audio in all the places you want it.
Adjust the levels of each audio track and the length of time it fades in and out by double clicking the track, then choosing "Audio" from the popup menu. Click "Done" when you've made the desired adjustments.
Play the entire track to check whether the loop sounds the way you intended.
IMovie only allows you to use audio files from your iTunes or GarageBand folders, so if your music isn't already in one of those locations, import the file to your iTunes folder before editing.
Keep in mind that iMovie only allows you to place audio over a piece of video, so if you try to drag the audio into the Timeline without any video present, it won't work. If you want your audio to play over a black screen, add in a black frame. Choose a Title page from the "T" menu under the Video Player, then delete the default text to make it a blank black page. Next drag this page to the desired place on the Timeline.