To enable autoplaying of media, use the Video/Audio widget to enable Auto Play (or Play on Page Load within the Media panel).

in5 Video widget (in5 > Interactive Widgets)
the in5 Video widget panel
InDesign Media panel (Window > Interactive > Media)the InDesign Media panel

Several browsers now limit audio and video playback that isn't triggered directly by a user action.

in5 uses an invisible workaround that captures navigation action (i.e. the user clicking to view the next page) and uses it to preload media to be autoplayed on page load.

This typically works for all except the first page (because the use hasn't clicked anything to reach the first page).

The Fix

You can workaround this restriction with autoplaying video, by using the Mute option in the Video widget panel. Muted video will bypass the browser's auto play restrictions.

You can see this fix applied in the video below.

Note that if you want to autoplay video with no sound and no controller (which can be done using the settings above), you might find that an Image Sequence is an even better fit.