Here's a very handy program which I consider one of my favourites, it allows you to convert back and forth through the extensions: WAV, MP3, WMA... of course with different sets of qualities for each one...
The wonderful thing about it is that it inserts itself as part of the right click menu, so u just right click on any of those audio files and a menu of Convert To: appears, allowing you to do it instantly, and of course u can select multiple files and make it convert them all together...
I use it sometimes to convert regular mp3's to a lesser size, 112 Kbps, which is not much different than the regular 128 Kbps mp3's, but it saves considerable space, specially when u want to put them on your cell phone or mp3 player....