Andrews,
Perhaps due to language differences we're confusing an internal audit with a root cause investigation. I generally think of an internal audit as something done 1-3 times a year company-wide to determine if the procedures and processes are ISO compliant and to verify that the procedures are being followed as they are supposed to be, etc.. It is probably not practical or advisable in most companies to do a full audit every time a customer complaint comes in. Most companies prefer to do an investigation specifically targeted at determining the cause of the specific defect/complaint. This is probably a more focused effort that does not waste time and resources looking at areas unrelated to the problem. Also, some larger companies or companies with higher defect/complaint rates may get 10, 20, 30 or more such complaints a year and it would be impractical and a waste of time/resources to do an internal audit for each one. Often the cause of a defect can be determined very quickly without lots of time/effort but sometimes the cause must be investigated on a much deeper level than a typical audit. You can still have have an independent person investigate or review the results of the investigation. JMO.