Our dog (that I thought didn't even know how to bark when I adopted him) has started boredom-barking at home during the day and it's driving me INSANE. We already put translucent material over the windows so he can't bark at dogs walking by, and we play calming music all the time. Both of those things helped at first, but now he just keeps getting louder.
A dog trainer would say: get him more exercise and then he'll be too tired to want to bark! Yeah, I know. I do exercise him a ton. But I want to do it on MY schedule and I don't want him barking while he waits for me! Also the more exercise I give him, the more fit he gets and the more he seems to require to be truly tired. I'M the one who is getting tired.