Unfortunately, after undergoing hundreds of hours researching on the possible solution, I finally found the answer in a book called: Pro Silverlight 4 in C# (Matthew MacDonale, APress).
According what is said on the page 436, although you can do it with Silverlight 4 (you have to write your own file header helper, store the row data as byte arrays and later on you have to convert them into a raw video data and what is worse is the audio and video are separated.), it is just not practical and worthy to do it. Because it requires reams of complex, handwritten code to convert it, and the conversion process is computationally expensive, which makes it extremely difficult to do in real time.
So I guess this would be the conclusion of my question. Now what seems still make sense for me is to find out why Microsoft doesn't support it and what is the new technology which is going to be used/ has been used to replace the SL.
Is it HTML5? But as far as I know, there is only a video tag in HTML which can only be used as a video player, but no tag to support to get the access of a web camera and save the captured video+audio at the same time as one file onto somewhere which could be a local hard drive or a network storage.
I hope all dear you could come up with some thoughtful advice.
I am going to leave this question un-answered for a few days to see whether there could be someone who can come up with some fantastic solution.
Thanks again.