You don't really need a plugin to do this. Just start an audio conversation and set your recording source from "microphone" to "wave" or "what you hear", then play your music in whatever mp3 player and the person on the other end will hear it, because the computer's recording source is set to digital audio rather than your microphone input.
To change your recording source, open the windows volume control (start->programs->accessories->entertainment->volume control), then click options->properties. Click the radio box next to "recording", and click ok, then click on the "select" box underneath Wave to use all currently playing digital audio as your "microphone"
. Some computers may call "Wave" different things like "Wave/MP3". Another option is "What You Hear", but that will send every single sound your computer makes, period...
