Some background:
http://hashihime.blogspot.com/2009/0...at-voices.html
Ritsu - Satou Satomi
Mio - Hikasa Youko
Tsumugi - Kotobuki Minako
Yui - Toyosaki Aki
That's the starting point, though I'd debate that the ED is played by the seiyuus (if it were, then that's amazing.)