你可不可以说得小声一点？Can you speak a bit more quietly. 你可不可以说得大声一点？Can you speak a bit louder? 你可不可以说得慢一点？Can you speak a bit slower? 对不起, 我不懂你在说什么。你可以说英语吗？Sorry, I can’t understand what you are saying. Can you please speak English?（actually it is a bit impolite, a better way is to say 我听不懂 instead of 我不懂你在说什么）

The first ,second and third sentence in Chinese is right,but the last sentence you should delete "吧".The right expression in CHinese is "对不起, 我不懂你说什么。你可以说英语吗？"

Thank you :-)

you did a great job.All of them are correct.XD