Ozzie barks at anything that walks by the door (when it's open), at loud noises, at other barking dogs, when the phone rings, when someone is visiting, when it's time to eat ... he is QUITE vocal. And when the mailman comes, oh God. Don't go there!
I know he hears lots of things that we don't. Some nights, he will lay there and just growl. Sometimes I never know at what. Other times, there will be a few minutes delay and I will hear one of my neighbors coming home so I know he heard them before they ever got out of the car.