Unfortunately, it's not possible to customize the volume of the soundtracks according to the different clips included in your project. Nonetheless, you're welcome to follow these steps to make sure the song interrupts while your video plays:
- Select and add one soundtrack from the Audio section, by tapping on the Music icon (a music note)
- The selected song will appear as a purple bar in the Audio section, right under the project preview
- Move your finger until the red vertical bar corresponds to the point on which you wish to interrupt it
- From the bottom menu, select Split: the selected song will be divided into two parts and you'll see two music icons appearing on the purple bar
- At this point, hold your finger on each portion and move them to the left and right, matching the beginning and the end of the video.