Found this brilliant free music file convertor.I wanted to change my wma files all to mp3 files and it did it easily and quickly.theres a few more other files it converts either way.wav, ogg etc. Go to www.download.com and type in "Switch 1.05" ( without inverted commas) It's a very small download, and so easy to use.Theres no adware/spyware, as I have checked with Webroot Spysweeper and Trend Micro.
Convert them to Ogg Vorbis (.ogg) instead of MP3. That way you get a smaller file size due to better compression while at the same time get higher quality than you would if used MP3.
And Matt_ uses good tools, LAME and RazorLAME are good products. LAME is known as the best. Everyone with knowledge about ripping, encoding/decoding use LAME.