NSVGUI is a open source adaptation of the NSVenc software written by NullSoft.
NSVGUI 7 beta 2 is pretty good
Current stable version can be found here
Also, ON2 has released a NSV compatible encoding plugin for the latest generation VP6.2 video codec.
This is free to download and use for "non commercial" use. Offers greater video compression and stability at low encoding speeds.
To maximize the audio portion of your stream, you should consider using AAC instead of MP3. AAC offers superior audio clarity and compression at low encoding rates - similiar to Ogg Vorbis.
StreamerP2P NSV set-up page