You need the WMReader or WMSyncReader object from the Windows Media Format runtime. They can only be used through COM InterOp. The open source WindowsMedia.NET library provides a .NET InterOp wrapper.
You can also use the WMAsfReader filter (which wraps the WMReader object) in a DirectShow graph (see DirectShow.NET).
MediaFoundation on Vista+ also provides its own ASF (aka WMV) reader, but that also requires COM InterOp.
MVP :: DirectShow / MediaFoundation <http://www.riseoftheants.com/mmx/faq.htm>