Just found smart audio converter from www.smartaudioconverter.com
It says that there's a free trial to download, but in the top right hand corner it says that it never expires. Could try that?
Or this one looks better from www.e-soft.co.uk
it's called audio converter / Audio grabber. This one allows you to batch convert your files. Be a lot easier than doing them one at a time.
Haven't used these myself but the last one sounds good.