At www.noambit.com you can find two download-able audio tours. One covers all the essential stops like the Duomo and the Loggia.The other points out neat little sights and places that many travelers never see.
Being able to travel at your own pace and do all or just a portion of a tour is a big plus during travel. All you have to do is put the tracks on your MP3 player and print out the map..then off you go.