Microphone issues (Siri, dictation & more)

I have an iPhone 6 plus with iOS 8.1.3 and recently I discovered that Siri can't hear me when I am not using a headset.

Unless I scream in the top microphone (near the flash) she doesn't register anything at all, and when I do she can't understand me anyway. The sound wave is moving all the time even when I am silent. With my headset plugged in Siri hears me fine and the sound wave is flat when I am silent. Filming with the camera on the screen side of the phone also behaves the same way, I have to be very loud for it to pick up anything but low static.

Filming with the back camera is producing clear sound which is louder when I speak close to the top of the phone, so the hardware is fine for the top microphone. Likewise the bottom microphone is working fine with google voice search in the "Google" app, the feedback on the screen shows more indication of sound when I am speaking into the bottom end of the phone.

Dictation doesn't work at all.

Voice messages in iMessage doesn't work at all, static here too.

FaceTime works, most of the time.

I have gotten FT to work with both front and back camera as well as with audio only. But sometimes it goes into the same kind of audio bugs where there is only static and you have to scream to make it pick up any sound at all. I think it is using both microphones. Voice seems to use lower microphone for phone mode, and upper for speaker phone. I can trigger something similar to the "bug" by being loud close to the upper microphone in speaker phone mode. But it resets back into working again if I am silent for a few seconds. It might just be an auto-level function trying to be nice to whoever I am talking to. In phone mode it does never cut off.

Snapchat only works with the back facing camera (upper microphone), just as the normal camera app.

Shazam works fine (uses bottom microphone).

Voice Memos works fine (uses bottom microphone).

Translate Voice works fine (uses bottom microphone).

Google works fine (as stated above, uses bottom microphone)

Since both microphones are working in other apps it seems to be software related. And it is only failing in some apps (Siri, dictation and iMessage voice messages, and sometimes FaceTime). Don't know if it started with iOS 8.1.3 as I almost always use my headphones, but I haven't noticed it before now.

I thought it might be a glitch in noise cancellation, but disabling it under Accessibility doesn't make Siri work.

I have tried turning Siri on and off again, changing all Siri settings (one at a time), restarting my phone, resetting my phone (10 seconds of holding down both the sleep/wake and the home button), resetting all settings. Nothing has made any difference yet. Any other suggestions?