There never will be. If I was to go with one theory it'd be that Haneke himself was sending the tapes, etc. some one goes into more detail on a forum somewhere. But no one will know, I doubt Haneke himself will spill it, if there is anything to spill.
shareLet's use Occam's razor. It was Majib or his son.
We should remove other options like Hanneke himself, Majib's wife(?), some Majib's friend who heard the story...
Both Majib and his son could be lying about not sending those tapes. That's an plausible option.
But who specifically? When Majib's son is arguing with Laurent in the bathroom, Laurent mentions that Majib wouldn't do it and Majib's son says that it wasn't him (he could be simply lying) and mentions that Laurent deprived his father of a good education (a reason!).
So I would say confidently that it was Majib's son. He had the motives and comparing to his father, more likely to put a camera on the street (or inside a car) and send sneaky notes.