Iphone5 mic problem

Hi all. I can't get siri too work unless I choose the raise too ear option in settings. FaceTime works fine and so does voice memos. If I was to make a video using the front camera then the audio is really quiet with lots of hissing and if I make a video using the main camera the audio is louder but still has a lot of background hiss. Has anybody got any ideas please.

I know the iphone has 3 mics so I'm thinking one of those might be faulty. I have already had to change the bottom mic/speaker/lightning port unit a few months back because there was no audio on normal phonecalls. It's an original iphone5 that's coming up too 2 years old so maybe it's time for a change.

I've not reset the phone in months and although I have a thin rubber case the problem is there with case on and off.

I'm wondering if siri uses one of the other mics to work. Maybe the one on the front next to the earpiece. Can't really find out much on the Internet and since my phones out of warranty I can't see apple being too helpful.

I found this video on YouTube of the exact same problem. This is what's happening with me

http://youtu.be/JlHtw9AhtWE