I also have the problem in the Rome mission with the sniper rifle in which it freezes on the hooker and middle guard with the audio not coming up. It's hard enough to get the sniper rifle onto a target as the sniper rifle and turrets are seriously unusable for me.
I haven't seen or heard anybody having a problem similiar to me, but when I first pick up a sniper rifle or turret to use the screen/view/crosshair moves rapidly on it's own without any movement from the mouse. If I hit Tab to go to the intel screen and then back to the game the view will stay still for a few seconds, at the very least (sometimes it will stay for good until I move the mouse then begin moving rapidy again). But if I let it continue moving on it's own it will eventually end up, I think, looking at the roof or something.
Decided to reload and head to Russia hoping that when I return I could have a solution to the problem. Almost done with all the Russian missions which leaves only Rome. I fear it will be the same when I return. Any ideas on what at least one of my two problems could be and how to fix it?