You can check $(document).scrollTop()
Try:
$(document).bind("scroll", function(){
if ($(document).scrollTop() >= 300) {
myvideo.play();
}
});
EDIT
Because We dont want the movie to play every time they scroll a pixel beyond 300 after the video is played you can unbind the event
$(document).bind("scroll.myScroll", function(){
if ($(document).scrollTop() >= 300) {
myvideo.play();
$(document).unbind('.myScroll');
}
});