Assalamualaikum. I want to ask that is it Haram/Halal to work in any audit firm? Such as KPMG, PWC.. as far I know that to work in any Bank which is based on taking interest is haram. But the audit firms are also helping the banks by auditing them. So, is it also haram to work in audit firm? Please someone help me by giving me a clear and direct answer.

I don't think it is Haram, as auditing is simple tracking and providing people with the actual truth and facts.
REgarding the banks, the concept of interest is said to be Haram, however in my experience Islamic Banks who say that they do not provide ineterest tend to do it in other ways under fees and other means.