Maybe the word “audit” is incorrect? If they didn’t provide you any guidelines, I’d definitely recommend asking. But it’s possible they’re just looking for your perspective on best practices and possible improvement ideas, more like a general code review.
There’s nothing quite like the unique pain of navigating an unfamiliar codebase that treats abstraction as free and lines of code in one place as expensive. It’s like reading a book with only one sentence per page, how are you supposed to understand the full context of anything??