Agreed. There is absolutely no excuse for Trump to let this occur. He should send his tech people in to physically end-to-end his lectern to ensure it can ONLY BE SABOTAGE, once such things occur. This unacceptable by Trump at this stage of the game.
They probably not allowed to check it out. They probably agreed to let the NBC sound engineers to do the mic check!
In other words, Trump’s motto for these debates should be: “Don’t trust, and verify”. Why the stupid Republicans allow this bunch at the Commission on Presidential Debates (a bunch of far left Clintonites and a few token DC establishment Republicans like Fahrenkopf and Danforth) to run the debates is beyond me. I would walk down the street with them only if I could keep my eyes on what they are doing constantly.