Conference XP is a project by microsoft Research that does just that
<http://www.conferencexp.net> (RTP over UDP). Their source code is under
the Shared Source license. Note that their implementation of the RTP stack
is not 100% compliant with the RTP standard and probably won't work with
other RTP implementations without some modifications.
I'm not aware of any other open-source C# implementation of RTP.
If you're looking for more general info about RTP, i'd recommend the
excellent book "RTP, audio and video for the Internet" by Colin Perkins.