My Ruthie does the same thing. I call her neighborhood watch. She's always on patrol if she's out front and even goes to make sure when people come home, she's there in their driveway by their car doors to make sure all is well. If a different car comes down the street...watch out, Ruthie is on it. lol.
Some are more 'mouthy' than others.... my girl, Banks, yep she would be one... bark, bark, bark, bark, bark. My Frenchie never barks unless he is playing. My other bully that passed, he also never barked unless something was truly amiss.
My Ella is very alert. Keeps her eyes on everything and everyone. Won't let me too far out of her sight, if I sit on the ground outside she has to come sit right next to me untill everything checks out, and no danger is present lol Shes almost 6 months old. She also barks at things that I can't hear lol
This is very normal. Ernie is becoming a grownup. He is realizing that he has the ability to change things in his environment. His world no longer revolves around his crate, his toys and his backyard. He has discovered the world!
My suggestion to help maintain your sanity is to teach him bark no bark. Using treats like you did to teach sit. Teach him to bark on command. Then teach him no bark. That way you have a way to get him to stop barking when someone comes to the door etc. Others who are much better versed on behavior and puppies will be along to give you some more ideas I'm sure.