Clay says that hitting play a second time "is one of the most frightening things i've ever done.what does he mean by this?Why is it so hard for him?

Clay is referring to the fact that the first time he hit play and listened to the tapes, he had no idea what was on them.... it was nothing more than curiosity. The second time around he was afraid of what he'd heard the first time..... it's difficult because he has no understanding what it's all about. He knows the girl (Hannah), but he doesn't understand the hostility..... and he isn't sure if the whole thing might possibly be a joke.