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iOS UPDATE (2014) might cause you this problem. Try first to reboot and if its still not working try the video.
So like i said on video i had this problem with my iPhone 4.
I searched up some solutions and i realized that the microphone of video recorder
You can take a needle and clear up that little hole next to speaker jax on your iPhone 4/4s.
Then try to record and comment me about any problems or questions.
Thnx for watching this !