Normally he's so loud but during lessons, he's as quiet as a mouse!
Normally she's so loud but during lessons, she's as quiet as a mouse!
Normally (s)he's so loud!
彼・彼女は普段は大声だ。
英語では「大声」"loud voice" と言えますが普段は形容詞の "loud" だけを使われています。
Normally (s)he has such a loud voice. とは少し不自然な気がします。
She's/He's as quiet as a mouse.
彼女・彼はねずみと同じくらいの音しか出さない。(ねずみみたい、とても静か)
なぜか英語で静かと言えば、ねずみです。とにかく、"as quiet as a mouse" はよく使われている表現です。
Normally he's so loud but during lessons, he's as quiet as a mouse! (男)
Normally she's so loud but during lessons, she's as quiet as a mouse! (女)
普段は大声なのにレッスン中は蚊のなくような声
私は小中学校で英語を教えていた間、このような子をたくさん知りました!