In the list of new features in Silverlight 4 you will find following:

Webcam and microphone to allow sharing of video and audio for instance for chat or customer service applications.

Silverlight captures an audio stream as raw pcm. So how would you realize for example audio/video chat or client/server audio recording application without any encoding on the client side, where there is no APIs in Silverlight available?

Much less in a Silverlight you cannot use an unmanaged dll. You can use a com automation (a new feature of the Silverlight 4, I think only for Windows) but only if it was already installed on the client side (do you know any encoding COM servers that are installed with the windows). Otherwise, how would you deploy a custom COM server within you Silverlight application?

The only way I found is either to deploy a command-line encoding and use it with COM AutomationFactory.CreateObject("WScript.Shell") or to implement an encoding to use it in your own AudioSink.