Were they more often devoted to New Testament subjects or to Old Testament subjects?
There are trivially googleable results, which if consulted, could allow this question to become a question worth answering: http://hcl.harvard.edu/libraries/houghton/collections/early_manuscripts/preaching/
Correct me if I'm wrong, but I don't think that preaching sermons really developed until the Renaissance with Girolamo Savonarola and the Protestant Reformation. Up to that point, I believe that Catholic services consisted exclusively of performing the Seven Sacraments. In Mass, I believe that, generally, only prayers would be said.