The phrase "you could hear a pin drop" means that the surrounding environment is so quiet that even a small noise can be heard distinctly. There are a variety of synonyms that can be used instead of this phrase, which include, "you could hear a whisper," "the silence was deafening," "you could hear a mouse fart," "you could hear a heart beat," "not a sound could be heard," "you could hear the rustle of leaves," "you could hear a sparrow sneeze," and "you could hear a fly buzz." These phrases effectively convey the stillness and quietness of a particular situation, which can be used in storytelling, journalism, and other forms of writing.