Today, a news blurb appeared that claimed that attackers could identify the location in which an audio record was taken by matching the mains hum to known data. Sadly, I fear that this might be feasible. The mains hum has quite a tight tolerance, but any aberrations (that cannot be done away with) could be spotted easily. In fact, power suppliers have to observe the hum and adjust their stations accordingly.
How could you manipulate a record such that this information is destroyed completely, so that it is not reconstructible by any means? Filtering alone might leave enough remains to still perform that identification. After all, all filters have a finite number of poles.