I'm skeptical of any claims that an emcee operates a sound effect or a clock. It is more likely that an offstage operator does that.
I can understand the skepticism. I had the Jeopardy! time's up podium button confirmed (and demonstrated for me) when visiting the set years ago before Alex's illness.
For all the episodes I've seen of Trebek hosting J!, I don't ever recall seeing him activating the time's up sfx, unless the directors really tried to hide it in camera shots. I have seen Jeff Probst numerous times hitting the button on R&R J! But I thought it was only because there weren't as many staffers on hand for that show (and under the guise of a different producer -- Scott Sternberg).
In some of the episodes of Bud Collyer's To Tell The Truth, I have seen him reach for the end-of-turn bell at his desk. I wonder if he received a cue offstage as to when to ring the bell.
And one of the more obvious ones i completely forgot about, as I consider them co-hosts...
Ruta Lee/Elaine Stewart were the dice rollers on the first incarnations of High Rollers. The contestant's fate was entirely in their hands in the main games.