What is being confused here is the role of the auditor vs the role of the SEC reporting/consulting function.
Assume PwC is the auditor, they would not be engaged in advising prospective transactions. Thats were you get into the SOX issues.
Assume KPMG is the SEC reporting auditor. They would inturn advise the company about SEC reporting. They could also be providing advice regarding acquitions and mergers. The company would also you other legal & tax firms to provide advice too.