Mr. Trump, checking your mic before the event is called "preparation." You failed.
As a young Captain, I worked at a high profile directorate at Fort Bliss, where new weapons systems were tested. It was a stop for virtually every VIP, as everyone was interested at what was coming online. 2-3 times a week, I gave the briefings, up to 3 star general and civilian equivalent. VIPs above that were handled by my boss, a full bird Colonel, though I was in attendance to answer questions.
Before every briefing, I made sure everything was ready, down to the temperature of the room, that all the lights worked, you name it.
Sure I know that this doesn't compare to last night, except that someone should have checked the microphones, and other details.