I was trying to get a silverlight media player webpart working and struggled to find the correct properties to get the video started. So I thought I would share the same with others who are facing same issue.
a) Add a silverlight webpart on your page.
b) Point the silverlight webpart to load mediaplayer.xap file from 14 hive under \layouts\ClientBin.
c) Set the custom initialization string property in the webpart to the the following in case you want the video to start automatically after loading. I placed the .wmv file in the same folder called ClientBIn. Hope it helps.