How do I change the position of the play button?

The position of the play button is defined at two places, first is the css file (.splash_play_button) and in the /modules/flowplayer-frontend.php (line 118, starting with $splash = …). To modify the vertical position modify top=round($height/2-45) to your custom value. Try for example values like 0 or $height to move the play button up or down.