Sorry for only coming back to you now and thank you for the information.
From what you have stated it does not seem that there is any substance to their new found audits, but merely an attempt not to pay yourself.
My advice would be to start the process to recover payment from them.
this should be initiated by way of a letter of demand. If no payment is forthcoming then to issue summons against them.
After summons you can immediately apply for summary judgement, as it seems that they do not have a bona fide defence to your claim and are merely trying to delay payment. This process can take up to 6 months, but with diligent actions can be finalised within 2 months.
should you not be successful with summary judgement, then the matter would proceed to trial which would entail oral evidence from yourselves and them. this process can take up to 2 years depending of the court were you institute the action.
I would advise that you employ the services of an attorney asap to prevent any delays in the process.
Please feel free to reply should you have any further queries.