Currently, I'm developing voice activity detection with the help of a sample program called Ozeki VoIP SIP SDK and I would like to share my experiences with you. This SDK is very flexible as you can implement it to already existing systems. You can use different developer environments such as Visual Studio 2010, Visual Studio 2008 or Visual Studio 2005. The support team helped me to configure the VAD to my actual needs.
So if you would like to check this site, you can do it here Ozeki C# VoIP SDK - A SIP SDK for software developers.