The English translation (source language) should be Please speak up (raise the volume), people in the back could not hear clearly. In Chinese, you can say 請說大聲一點，後面的人聽不清楚。Or 請提高說話的音量，後面的人聽不清楚。 Hope it helps.

请把说话的声音提高一点， 人们在后面 听不清楚了。 Correct. But it doesn't sound authentic or idiomatic. Maybe the better is : 请你声音大一点，后面的人听不清。 请提说话的声音高一点儿， 人们在后面听不清楚了。 Wrong.