Siri or microphone issue on iPhone 7 Plus

A few weeks ago, I noticed that Siri could not hear me properly. First, I thought that it was some sort of temporarily hick up. But after a few days it was still a problem and now it started to annoy me.

When I use a headset (Lightning or Bluetooth) Siri understands my just fine. But without the headset it picks up less than half of the words I say.

This is what I already tried:

1. Turn phone on and off
2. Turn Siri on and off
3. Hard reset phone
4. Install iOS 12 beta
5. Clean install iOS 11.4 without restoring from back up.

To test the three separate microphones, I made a video with front facing camera and the camera on the back. The microphone on the bottom I tested with the voice memos app.I also tried to cover up the other microphones. Just to be sure the recorded sound was coming from the right mic I wanted to test.

All the microphones seem to work fine, but somehow Siri does not hear my properly without headphones.

My one-year warranty from Apple has expired. I do have two-year warranty from the shop that I bought from. But I want to be sure that it is hardware related.

Is there something else that I can try?