This appears to be an issue in iBooks (for Mac), when embedding an iframe within an iframe. If you use an HTML widget, iBooks for Mac will not play the video, but only the audio. (A bug)
I recommend using a standard rectangle, and pasting the Youtube embed code within the inner HTML of that element directly. The video plays correctly in iBooks for iPad, even if it is a 2 levels-deep-iframe.
So to get better compatibility, your embed code would look like:
<iframe width="560" height="315" src="http://www.youtube.com/embed/MfPwa-oIUmI" frameborder="0" allowfullscreen=""></iframe> (And use a rectangle)
Hope that helps! And welcome to Hype.