I am having the same issue. I just downloaded “mavericks” software for the OSX upgrade. And also have been playing around in Garage Band… not sure if it is related to either but it happened after doing both of these things.
The Sequence settings shows the audio to be 48k, and unchangeable anyway.
If anyone knows of a way to go back to my old software I can do that and see if that solves the issue.
Ken