Using this software you may convert WMA file to MP3 for free. Also, you can extract audio from WMV and ASF video files and save them in MP3.
Here is a short tutorial:
Start Free WMA to MP3 Converter
Add files to convert. To do it press Add file (first) button on toolbar.
Select settings for MP3 format (it is located at the bottom). You can select among predefined or defines your own.
Choose output folder where converted MP3 files will be placed.
Now everything is ready for conversion. Press Start (second) button on toolbar and have a rest while conversion is in progress.