To download from Netscape ("save link as") you need to press on the shift key and then click on the link with button1.
The RTP home page contains useful information and links to RTP tools for help in debugging your application. In particular, rtpdump and rtpplay can be used to verify correct packet decoding and generate test traces. There currently is no tool that converts a .au file into an RTP stream (although I would welcome any contribution....).
NeVoT can generate RTP streams from .au files, while the other audio and video tools generate streams from live inputs.
RTP is defined in RFC1889 and RFC1890. These RFCs contain all the relevant information you might need.