My teacher has such a quiet voice, it's hard to have an English class with him.
My teacher speaks so quietly, it's almost impossible to learn English with him.
この場合、次のような言い方ができます。
ーMy teacher has such a quiet voice, it's hard to have an English class with him.
「先生の声がとても小さくて、英語の授業にならない」
it's hard to have a class で「授業にならない」と言えます。
ーMy teacher speaks so quietly, it's almost impossible to learn English with him.
「先生の声が小さすぎて、英語を学ぶことが不可能に近い」＝「英語の授業にならない」
it's almost impossible で「ほとんど不可能・ほとんど無理だ」
声が小さいことは to have such a quiet voice や to speak so quietly のように言うのが自然で良いでしょう。
ご参考まで！
The new English teacher's voice is way too quiet. This is not an effective English conversation class.
On top of that, he/she is wearing a mask and sitting down during class.
Most students have no idea what he/she is saying.
ご質問ありがとうございます。
"The new English teacher's voice is way too quiet."＝「新しい英語の先生の声は小さ過ぎる。」
"This is not an effective English conversation class."＝「これは効果的な英会話の授業ではない。」
"On top of that, he/she is wearing a mask and sitting down during class."＝「その上に、彼・彼女はマスクをして、授業中は座っている。」
"Most students have no idea what he/she is saying."＝「ほとんどの学生は彼・彼女が何を言っているのか分からない。」
ご参考までに。
カナダ 
アメリカ合衆国 
