http://www.mediahuman.com/audio-converter/ - Free. Says it does WMA. I have not used this so try it at your own risk..
Switch (free basic version) - http://www.nch.com.au/switch/index.html - I'm not sure if this will do WMA
All2MP3 - http://download.cnet.com/All2MP3/3000-2140_4-190407.html - mixed success with WMA
Flip4mac - need the intermediate payware version which lets you convert. Use with Quicktime player. Flip4Mac - http://dynamic.telestream.net/downloads/download-flip4macwmv.asp?prodid=flip4mac wmv - The free version will let Quicktime play but not convert Windows Media files.
After installation a new Flip4Mac pane will appear in System Preferences. Make sure you set the boxes to ensure that it will play WMV files in Quicktime.
VLC - not really a converter but will do some in the export menu. May be slow at converting and there will be a learning curve.
Explore online converters where you upload a file in one format and download in another.
I'm not sure if your computer can run Audion 3 I think I used to use it to convert WMA.