Jan, when I looked into it years ago, a subscription audit was a check by an
outside firm of how many copies of an issue or magazine were actually sold, by
subscription, from store counters, etc. and was used to determine cost of
advertising by firms that advertised in the magazine.  For example, if TIME
magazine printed x copies of an issue, how many went to subscribers, how many
to dentists offices, libraries, etc, or multi reader sites, and how many went
straight to the pulp mill?  Advertisers want to know, before they pay the per
page costs of a full page ad.  Therefore, the organization to which the lady
you called referred is probably something like the National Advertising
Council in some form.  Hope this helps.  John Piety, John Carroll University.