No voice front camera

Hi,

I have an iPhone 5 but when I record a video with my front camera I can't hear my voice just some other noise. Also when I call someone in speakerphone mode they can't hear me. When I record with the rear camera and the voice memo app everything is normal. I don't want to send it to Apple so I want to repair it by myself but I am not sure if it is the front microphone which is defect or one of the other two. I hope you can help me.