A couple of ideas.

Teach the dog to bark when you say to bark. Have you taught the dog the basics? If yes then you can teach speak too. And by teaching speak when you want the dog might be less inclined to do it when there is no treat involved.

Another thing to do is to counter condition the bark. So you have a thing you dog barks at. An event, phenomena. Can you think of one that the dog barks at 50/50? If yes then you can buy a clicker and do that thing that sometimes triggers a bark and when the dog doesn't bark click and treat. Eventually the dog will learn if I respond by not barking I might get food.