HOW TO UNMUTE FACEBOOK VIDEOS
ADD &mute=0 into facebook video embed code
EXAMPLE:
<iframe src="https://www.facebook.com/plugins/video.php?href=https%3A%2F%2Fwww.facebook.com%2Fearthwonders12%2Fvideos%2F490676221419200%2F&show_text=0&width=380&mute=0" width="380" height="476" style="border:none;overflow:hidden" scrolling="no" frameborder="0" allowTransparency="true" allowFullScreen="true"></iframe>