Given voice replication technology and reported cases of false testimonies (posted elsewhere on this forum), I'm not sure what's up with this. There seems to be no public information available, that I can find anyway.
'the authenticity analysis of audio microcassette recordings in the Canadian Prime Minister Stephen Harper vs. The Liberal Party of Canada civil case'
http://www.bektekllc.com/documents/Bruce_Koenig_CV.pdf