Why can't i hear the sound in my iphone 4?

Okay so i can hear like ny ringtones and stuff but when someone calls me i can hear their voice but when i talk they can't hear mine. I was screaming too. And when i record videos uou can't hear my voice either. I haven't dropped my phone and haven't gotten it near water. What do you think is wrong?

You are going to need a new phone.