Background checks can go awfully awry, they can get fixed, but I would not quit until you are officially hired.
The trend of late is no notice at all, which only means you will be blacklisted from working for Nuance and getting blacklisted from Nuance would not keep you from working anywhere else.
They have to pay you what you earned. That is the law. It might come a little slow, but they have to pay you including PTO earned.
Before you quit, be sure to have all the information you need on Cobra, etc., because they could get nasty about not providing the information although by law they have to do that.