The neighbors are far away. How could they hear it, but not him, since he would have been awake at that point as he observed them outside and subsequently leaving?
Your time sequence is mistaken, and a better sequence makes sense.
CB was asleep. As he slept, there was a scream. The dogs started barking. He awoke to the sound of dogs barking, and peeked out the window to see why. There he saw Holly's car still there, and he saw Holly talking with man (he didn't get a good look at them but who he assumed to be her bf) in the garage.
Later he looked again and saw them walking into the woods side by side.
There was no scream that occurred from the time he awoke to the time they were (as he saw it) talking a stroll.