Currently the video does not play, only audio is heard. If autosizing is disabled the video functions properly.
A possible workaround is to disable autosizing and manually call the autosize() method when the page is loaded for all other browsers:

<telerik:RadWindow runat="server" ID="rw1" VisibleOnPageLoad="true" NavigateUrl="PlayVideo.aspx"
    AutoSize="false" Width="700" Height="500" OnClientPageLoad="OnClientPageLoad">
</telerik:RadWindow>

 and

function OnClientPageLoad(sender, args)
{
    setTimeout(function ()
    {
        if (!$telerik.isMobileSafari)
        {
            sender.autoSize(false);
        }
    }, 0);
}