iPhone 5S mic not working in facetime or video recording with iOS 9.3.2

Just over a week ago I bought a new iPhone 5S and after a few days, I Facetimed my girlfriend, but after a second or two my microphone stopped working and she couldn't hear me. My microphone wasn't muted and she could hear me fine in the first second. I tried her Facetiming me to see if that made a difference, it didn't. I tried Facetiming my friend to see if the problem was my girlfriend's phone, it wasn't. I then tried both my parents and they couldn't hear me either. My microphone itself is working because I had phonecalls that went fine and I could use Voice Recorder.

So I went into a chat with someone from Apple and after checking my settings, they got me to reset my phone back to factory and get rid of all the settings. I did that and it still wasn't working so they told me to bring it back to the store I bought it from. I brought it back to John Lewis where I bought it and had warranty and explained what the problem was and they immediately ordered a new one for me, which I collected today.

I brought it back and tried Facetime because I didn't want to put my own screen protector on it if the same issue occurred, as I had to with the first one. Again, my Facetime audio wasn't working when I called people, I could hear them again but they could not hear me after a second or two. This made me think it was a problem with what I was doing. However, I saw someone else complain about a similar situation and their microphone didn't work in video recording either. So I tried video recording and all it recorded sound-wise was static.

If anyone could help it would be greatly appreciated, I am sick of having to chase up a phone that will work. This kind of situation makes me want to return the phone and just buy a Samsung.