This is a unified diff (you probably need GNU patch to apply it) to lame 3.58. Please note that this diff is now obsolete as recent beta versions of lame include a version of this code. Just type "make mp3rtp". The code was broken, but the CVS code is reported to work (and interoperate with playRTPMPEG) as of Feb 21 2000.
Please note that the output stream will only have the correct speed if the input is live recorded stream from your sound card or lame encodes exactly in real-time on your CPU!
RTP is the Realtime Transport Protocol as defined in RFC 1889. It is the transport mechanism of your choice to multicast an mp3 stream.