Tbh, I dont think you can place much stock in auditors. Most famously, Enron where the red flags were flying at full mast and Arthur Andersen were completely asleep at the switch. Similar with Lehman Brothers (Ersnt & Young) with their highly irregular financing practices that grossly distorted their balance sheet.I had a quick read through the Financial Reporting Council's review of BDO's conduct in that audit. The issues were much more complex than ours would have been.
Essentially an audit is about managing risk. You can't possibly track every penny in and out so you devise an audit plan to mitigate that risk as far as possible. You work out what is material and what isn't. If a payment doesn't match an invoice by 20p, then you wouldn't care about that.
As an auditor, you'd want high percentage coverage of the largest revenue sponsorship contracts, with decreasing coverage of lower value contracts. So you might dig into all of the highest ten revenue earners, half the next ten and just randomly select a handful of the smaller ones. You'd want to check the contracts fully and ensure cashflow matched the terms of that contract. You'd also want to check that the amounts received had been correctly accounted for and were represented correctly in the accounts themselves.
The issues at NMCM were complex and around contracts where there could be some variability in what revenue or potential losses could be reported. BDO seemingly devised a reasonable audit plan but didn't follow some of those plans through properly, and didn't challenge the management view of these robustly enough.