speak up        
phrasal verb with speak verb uk/spiːk/ us/spiːk/ spoke | spoken
B2 提高嗓门,大声点说
to speak in a louder voice so that people can hear you
  Could you speak up? We can't hear at the back. 你能大点声说吗?我们在后面听不见。 
 - More examples
 - You'll have to speak up, I can't hear you.
 - Stop muttering and speak up!
 - You'll have to speak up a bit if you want everyone to hear you.