I've never heard of it happening.
I've participated the vetting process used to make the assignments for Cir Assm & Dist Conv parts. These guys have to be diehard Dubs to get chosen, but granted there is evidence of some "moles" here that have had some pretty high credentials to be leaking what we get.
And, YES, the Conv Chairman would have the sound turned off. There is always another diehard Dub assigned to follow the speaker and compare his words with the outline (or manuscript). He is supposed to be ready to "step in" if the speaker faints or dies or "goes off course". Anything more than just a minor variance and he'll be in a gestapo meeting as soon as he's off stage. And, of course, these talks have been rehearsed in advance and heard by the CO or DO (back when) or Conv Chairman.
It would sure be fun to witness that though.