In this post, Mike Ormond discusses microphone access from
Silverlight.
Source: Mike Ormond's Blog
Access to the microphone is provided through the Microsoft.Xna.Framework.Audio namespace in the Microsoft.Xna.Framework assembly. You’ll need to add a reference to this XNA assembly from your WP7 Silverlight app if you need microphone access.
The class we’re interested in is Microphone. This class provides access to all the available microphones on the system and exposes a static property – Default – that returns the Microphone instance for the current default recording device.