Los Bravos was a Spanish pop group with a German lead singer who was signed by the Spanish division of Decca Records and traveled to England to record. Brits Tony Hayes, Steve Wadey and Michelle Grainger wrote a song Los Bravos recorded called "Black Is Black". The song entered the pop charts in England at the end of June, 1966.
My research strongly leads me to believe that our Paul's real last name was BLACK. So I went looking for--and found--some amazing backmasking in "Black Is Black".
First, the song's lyrics are about a boy who loves and loses a girl. The upshot is that the love might be "bad" but he wants her back.
Second, for those who haven't followed the discussion about the name of the man who replaced our Paul--the consensus is that the man's first name is William, and he was commonly called Billy or Willie.
So, beginning at 0:11 in the reversed song and repeated a total of five times throughout the song is the following:
"Willie thought, 'Ha, ha, ha, ha.'" [!]
The date of the backmasked message implies that Paul was on his way out of the group by June, 1966.