Ahh, I didn't get it at first either.
When they hit the ball with the baseball bat, they hit with such a force that it makes a sound like thunder, so when the humans hear it, they won't think anything of it and will pass off the sound as thunder.
Hope that helps...
When they play the sounds they make hitting the ball or each other in Emmett and Edward's case, the noise sounds like thunder. It would be suspicious if there was thunder but no storm. It's a cover.
the answer cause when they run into each other/hit the ball it sounds like thunder and it would be wierd if in a perfectly lovely sunny day all of a sudden you here thunder! also 1. duh! haven't you read the books? 2. if you haven't why r u watching the movies? you miss so much and it makes so much more sense