That's a big can of worms.
I'm no legal expert - but considering all that was said on there about voting machines & Dominium, it would seem like a slam dunk having Hannity say this. But then I am sure he was well briefed before going to answer questions.
But in business defamation/disparagement claims in the US, you have to prove loss of the amount you are claiming. Did Dominium really lose $1.9 Billion dollars because of these statements? I would wager not. Their machines are still being used in the US, not sure about rest of the World.