Short answer: iOS doesn't support webm videos.
http://caniuse.com/webm
I haven't worked with Joomla in ages, but you're going to need to either find a module that will support cross-browser HTML5 video, or implement something like this yourself:
http://www.jplayer.org/
Unfortunately, regardless of the solution, you'll need to re-encode your video into three formats: MP4, WEBM, and OGV.
More info and some example code here:
http://designfestival.com/implementi...back-in-html5/