Actually, I think you don't need to modify prefs.js as what can be played with Media Player can be played directly with the plugin. However, there are some embedded types that the plugin cannot handle, not even in Mozilla or Opera.
I once wrote a text about using WMP in K-Meleon, including some test results and other advice.
I think the file is still up at:
<a href="
http://www.bahamapress.org/andres/wmp.zip">http://www.bahamapress.org/andres/wmp.zip</a>