From the website of (I think!) an Italian early-music singing group,
They've made many mp3s from their repertorio available on this page. They are categorized as canto gregoriano; monodia e polifonia medievali; and polifonia dal Rinascimento a oggi, with subsections composizioni a cappella and composizioni vocali e strumentali. (Even I can read those!)
They're pretty good, too. Here's a sample of their sound: the Gregorian Hymn Urbs Jerusalem. Here's Omnes de Saba, a gradual for the Epiphany mass.
Many of the files are only samples, alas.