Several browsers/devices restrict auto play without user interaction.
in5 has an automatic mechanism to record interactions and use them to silently preload media, but it doesn't go into effect until the user interacts (e.g., on the first page that loads), so auto play won't work until then.
You can use the Video settings panel (in5 > Interactive Widgets > Video) to mute the video, and this will bypass the browser autoplay limitation.
