Does this help?
http://www.google.com/search?source=ig&hl=en&rlz=&q=C%23+ogg&btnG=Google+Search&aq=f&oq=&aqi=g3g-m1
Actually, going through those results is what I had done a while back, and it's shockingly poor for what it looks like on the surface. Basically, all of the C# ogg decoders are pretty old and/or incomplete, or don't really work, etc. What I'm using is this:
http://oggvorbisdecoder.codeplex.com/That's a C# wrapper for the official C/C++ ogg libraries, and it's quite well done. The original ogg libraries are 32bit only, I should warn you, so if your application tries to P/Invoke into those from a 64bit architecture when compiled to Any CPU or 64bit CPU, it will fail. So you'd want to either compile your application to 32bit (which I have done for AI War for a variety of performance reasons, and of course it still works great on 64bit systems), or you'll want to recompile the basic ogg libraries for both 32 and64 bit versions.
Or if you want a native C# solution, you'd probably have to code it yourself. As I recall, the source for the official C/C++ libraries is open source, so it shouldn't be that huge of a hassle. Was more of a hassle than I wanted to go through at the time, though.
