The St. Ann Choir website has reposted Susan Altstatt's chant pages for the first Mass of Christmas, which are shown here. I copied them because the images are not linkable directly (you have to go to www.stannchoir.org and click on Music to see them, I think because the site uses frames).
NOTE: Copied with permission. Can be displayed online but not printed without permission. For how to obtain permission for other uses, please contact Susan: email@example.com.
In Galli Cantu
Ecce Virgo Concipiet