Maybe search a scripting site for it. If you don't find a script for it I suggest you try a web search - Search for audio conversion . There are a lot of programs out there to do what you need.
why not just have ur mp3 player recognize wma files and use windows media player to play them thru ..... thats what i did on mine to work around that issue for my gfs having a ton of wma and hardly any mp3 .....