How did they so badly miss the punch sound effect in that one punch Sonny hit Carlo with? He didn't miss! He throws the haymaker, Carlo's head snaps back yet no sound effect! Has that ever been discussed?

Yep it has been discussed many times. It is one of the worst mistakes in the movie, and sadly it is made where great attention to detail is paid, namely the baseball game to which Carlo is listening and the fight poster on the wall behind the scene where Sonny is beating Carlo to a pulp.

I also think that the actor who plays Genco Abbandando in GF2 is an onlooker (extra) during that fight scene. Or maybe it is just some guy who looks like him.

Yes, it's the same actor: Frank Sivero. He also played Frankie Carbone in "Goodfellas."

True TB. But do you remember his appearance in an episode of Happy Days?

I think it was better without sound effects. I never noticed it till reading this thread. Perhaps Coppola tried it and it did not seem as good as going without them?

I always thought the scene where Clemenza kills Carlo in the car is poorly done sound wise. The audio doesn't match what is happening as far as his kicking of the windshield, and the sound of the murder also doesn't sound like you would expect it to. It sounds tinny and fake.