Here's what I did with my new rescue : When she first barked at anything, I acted all alert and said "What's that?"
Then when she looked at me w/ my shocked/scared face I said it again, "What's that? She may or may not bark right then.
Keep doing this for every little bark your dog does.
It helps if your dog pays attention to your facial emotions and voice inflection. They pick up on the nuances.
Within a few days, whenever I heard a noise outside, I'd ask her, "What's that?" What's that? in a worried voice.
She picked it up real fast. They will learn quickly what I call their BEAR BARK, deep, mean sounding bark.