You can set the animations on a scene to follow the timing of a video or a sound you previously imported on your page.
Here is how you use this functionality:
Once you uploaded the media to your library, select it with the + tab on the top left menu in the editor
You can now find it in the list of your elements/layers in the timeline
On the timeline, next to the name of your media, you have a clock-shape icon
Click on this icon. The element line and the icon itself are now colored in pale blue.
Your media is now in master mode: all animations on the page are set on your media's timing
Important: only video/sounds that are fully uploaded to your library can be used as master media. If you added a YouTube video URL to your library for instance, you won't be able to set it as master media. The clock-shaped icon will not appear on your timeline.