They actually study medieval non-religious music and try to reproduce it based largely on the bitching of ecclesiastics about it. For example, if a cleric complained about horrible bray strings, they add bray strings. Their web site also describes the instruments they use and gives sound clips:
http://www.corvuscorax.de/start.php?lang=EN Me, I think bray strings are the bomb, but I think everybody knows that about me by now.