Site is using a large number or embeded audio clips for quizzes. Prior to the update to 1.8.8+ the quiz would show an audio player which students could listed to before attempting the quiz answers. After the update, the media player disappeared. On swapping out the new lib.php (1.66.2.23) for the old one (1.66.2.20) the player re-appeared and worked again. No error messages in the server logs or in debug that I could see. Couldn't test extensively as it is a client site.