video without sound on iOS

I'm using the html 5 video tag, but when running on an iOS (an app using the phonegap) there is no sound, what can it be?

I followed exactly the example used by W3SC, and even then there is no sound.

Code:

<div class="midia">
    <video id="video" controls preload="auto" autobuffer style="border-radius:1em;">
       <source src="http://www.w3schools.com/html/mov_bbb.mp4"type="video/mp4">
    </video>
</div>