Have you updated your audio and video drivers
? Being a Ubisoft game, I would suggest downloading the patches and then installing them (if there are any), for the most part, doing so automatically creates more problems than it solves. Also, if you are only playing the Campaign I would suggest not bothering with the patches as again, they break more things than they fix... my personal opinion of course.
Another thing I am going to suggest is going to the AC Ubisoft forum to see if others have the same issue and perhaps a solution for it.