Fullscreen background videos that autoplay right when the webpage loads (above the fold) has become quite a popular trend these days. Personally I think a fullscreen good quality video that autoplays does increases engagement for the users/customers. It should be kept in mind that the story of the video must be relevant to the brand. These days we’re surrounded by loads of videos on social networks like FB and Twitter as well which autoplay (but is muted of course). Analytical studies have also reported higher engagement due to this.
It is quite easy to get the playback length (in seconds) of an audio or video file. The file may be in whatever format that the browser supports – mp3, mp4, ogg, wav, etc.