Audio problem: When Benny and the others are looking at the destroyed baseball, when Benny and Kenny are talking their mouths are out of sync with what they are saying.
Audio problem: When Yeah-Yeah goes over the fence, he's panting but his breathing doesn't match his mouth or shoulder movements.
Answer: It's a blue hat with a yellow/gold capitol "C" for the logo, which doesn't correspond to any major league teams. It could be a minor league or school team hat. Benny refers to it as his "old" hat, so he may have replaced it with the Dodgers hat he's wearing.