She looked as if to say, "what on earth is that sound?!"
She had this look on her face like, "What is that sound?!"
ご質問ありがとうございます。
① "She looked as if to say,"=「彼女は~と言うように見えた」
" 'What on earth is that sound?!' "=「あの音は何?!」
② "She had this look on her face like,"=「彼女は~のような顔をしていた」
" 'What is that sound?!' "=「あの音は何?!」
☆"What on earth is ~ "は "What is ~"よりも驚いた感情を強く表しています。
ご参考に。