go to your library/audio/midi driver and check if you have an m-audio firewire410 driver or any m-audio driver delete that driver, for some reason it affects the DAC driver, i dont know why but it works..
this question has been addresses but not answered, i use logic7 express and ive reistalled my os 3 times , cause my midi driver in osx will dissapear , i dont know how this happens but i know when it does cause when i go to load logic it will load everthing to open up and when it goes to check...