DHau - "speak" was one of the hardest things I've taught my dog, because she is naturally just so very quiet unless she is herding. She had a lot of moans and groans, but never barked. The only bark she would give was at herding, and it's sort of a yippy bark that, on more than one occasion, had people around us asking, "What dog is making that sound?" because they could not believe that a Shepherd could make that sound.
It took me forever to get those first few barks out of her so I could praise and reward for the behavior and build it up to where it was consistent. I've found that if I act really really really excited - as in, jumping up and down, waving some really good treats, flailing my arms, and just generally acting like a fool - I could eventually get her to give me something like a low "woo". Nowhere near a bark, mind you, but enough for me to click and treat for it, and then try to repeat it and see if I could get more of a sound.
I did eventually get something like a bark (a kind of, "Uh ... wait ... was that me? Did I just bark?" kind of bark) and I marked and praised for that.
Is there anything that sets your dog off barking or gets him/her super excited? For mine it's going for a ride in the car. So using your treat and excitement together with something that stimulates a bark may work well for you.