Here are four public domain Gregorian Chant music clips for you to download and use in absolutely whatever way you want. Public domain means no one owns it, it means you can just use it with no restrictions. These are pretty good quality clips too.
Gregorian chant is the central tradition of Western plainchant, a form of monophonic liturgical chant in Western Christianity that accompanied the celebration of Mass and other ritual services. This vast repertory of chants is the oldest music known as it is the first repertory to have been adequately notated in the 10th century.
Gregorian Chant 1 (right-click on this link to download)
Gregorian Chant 2 (right-click on this link to download)
Gregorian Chant 3 (right-click on this link to download)
Gregorian Chant 4 (right-click on this link to download)
If you need a bigger variety of royalty free classical music for your project check out the Apollo Symphony Orchestra: Greatest Hits collection of 50 of the most recognizable classical music tracks available for use in your commercial projects.
The collection above comes with a free download of 25 royalty free Tchaikovsky classical pieces.
For an even bigger variety of royalty free classical music check out the Royalty Free Classical Music section of musicloops.com.