HMM Why not just use the mp3 codec plugin? It installs the Fraunhofer Pro Codec then all you have to do is chose the mp3 codec and bitrate in wmp settings and then as an extra bonus in audio codec proerties you can go to the Fraunhofer Pro Codec and chose from Stereo, Jonit Stereo or Joint Stereo IS/MS.
It encodes excellent and to too terribly slow either.