不知你要问啥?我把句子分析给你,希望你理解赞同!
what frightened us most of all= the thing(that frightened us most of all)
was that not a sound(=no sound) could be heard that not a sound could be heard 为表语从句
全句意为：最使我们感到害怕的是听不到一点儿声音.
希望你能理解和赞同.
很乐意与你探讨英语问题!祝你不断进步!