http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnwmt/html/WMA9_Multichannel.asp
"Learn about the multichannel and dynamic range control features of the Windows Media Audio 9 Professional codec and how the codec transforms multichannel audio content into stereo audio content when necessary. This article shows how a programmer using the Windows Media Format SDK can create an encoding application that enables a content author to determine how that transformation happens."
