When Mannhattan was added to the game, Valve recorded new lines for all the classes when Robots would attack gates. For some reason they coded these lines to ONLY be used when you're on Mannhattan and on other maps, the mercs would simply say nothing at all.

With the power of a couple of trigger_multiples and some creativity, we can workaround this and grant the mercs the gift of gab that they were originally denied!

You could use this same method to make people say anything, even some of the unused MvM talk such as wave clear or attack Tank talk (as you may have noticed if you've ever played Dockyard/Shiver Peak)