whether you're using a Mac or windows, go to Preferences, click on Advanced, go to Importing, set encoder to Mp3 encoder. then go back to your library. choose the songs you want to convert to mp3s by highlighting them. right click and choose "convert selection to mp3". voila. The original file will stay in your library, either as an AAC or lossless but the new mp3 file will be added. so depending on the number of files you want converted, this may eat up your hard drive's memory. Note further that mp3 (with 160kbps) files take up more space than a 128kbps AAC file of equivalent sound quality.