I went to mass this morning at the Duomo. They were singing (chanting?) Gregorian chants (I guess chanting...) for about a half an hour before the service started and it was incredible. The acoustics were amazing and the chanting went in this circular kind of pattern and echoed beautifully. The mass was in Italian, which I can't completely follow, and I'm not really into religion anyway, so I spent most of the time people-watching. Very worth it.