because people speak English
A) Tell them that you can't hear them.
I would use one of these sentences to do so, depending on the atmosphere of the situation!
I beg your pardon.
Could you speak a little louder, please! or
Have you got a frog in your mouth?
It's a big problem to me that I can't hear someone clearly so I will shout whaaaaaaaaaaaaaaaaaaaaaat????
Depending on the circumstances, I usually act in (A) or (B) manner, for example depending on speaker's gender!.. :))
I ask them to speak up or repeat what they said. It's really bad if you can't hear someone and pretend that you heard them. It's really bad!
Beg your pardon? - for example.