Hello, other than checking the Silverlight client, you should also check the server. If you're using Windows Media Services, there're a few restrictions regarding mp3. The following statement is from http://www.microsoft.com/windows/windowsmedia/forpros/server/faq.aspx#2_6.
Why can't clients stream some of my MP3 files?
Windows Media Services cannot stream multiple-bit-rate (MBR) MP3 files. If the MP3 file was encoded by using a multiple bit rate, it will not play back as expected when it is streamed from a Windows Media server.
In addition, if the MP3 file content is too short, the server may not be able to stream the file to Windows Media Player. To ensure reliable rendering, the content must meet the minimum supported content length of the player to ensure reliable playback. The minimum supported content length for Windows Media Player 9 Series or later or a player that uses the Windows Media Player 9 Series ActiveX control is 5 seconds. The minimum supported content length for earlier versions of Windows Media Player is 30 seconds. Content that is shorter than this minimum length may not render completely or at all.
shanaolanxing - I'll transfer to the Windows Azure team, and will have limited time to participate in the Silverlight forum. Apologize if I don't answer your questions in time.