if you can, it won't be in a way that works totally cross-platform - iOS for sure does not allow media to be autoplayed (quite right too) - and I'm sure other platforms enforce the same rule.
It's nothing to do with the _setCompletionOn property though - that defines when that component gets set to completed: when it scrolls into view or when the user actually clicks play.
I don't think there's any way of doing it through Adapt at it is - but you could probably amend the contrib-media component to get it to invole the play() method of the media player when the component scrolls into view... inview is the function you'd be looking to hack: https://github.com/adaptlearning/adapt-contrib-media/blob/master/js/adapt-contrib-media.js#L58