If you are working in Office 365 and wondering where the silver light media webpart. Well, it's not there out of box. You could get that working with the following steps :
1. Add the Silver Light WebPart to page.
2. Configure Edit Webpart -> URL to -> /_layouts/ClientBin/MediaPlayer.xap
(The path is now relative to where the XAP is loaded from - as in the MediaPlayer by default will want to find your media file in the ClientBin dir.)
3. Set the initialisation string to something like:
mediaSource=../../WP/Videos/wildlife.wmv, displayMode=Inline,mediaTitle=shopSharepoint.com did it for me,autoPlay=true
You should be all set.