Are there any good wma to mp3 converters?
How can I mass convert audio files from WMA to MP3?
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.
SoundConverter can convert from any format that you have GStreamer read plugins for, and to several popular formats.