Are there any good wma to mp3 converters?
How can I mass convert audio files from WMA to MP3?
SoundConverter can convert from any format that you have GStreamer read plugins for, and to several popular formats.
FFmpeg is another great option. You can compile it yourself, or grab the Windows binaries from http://ffmpeg.arrozcru.org/autobuilds/.
Assuming all your wma files are in the same directory, you can run a batch command like
in the directory containing all your wma files.