iPhone 6s bottom microphone with low sensitivity

Many months ago I've send my iPhone 6S back to Apple to get the battery replaced, under the free battery replacement at that time, and Apple gave me back a refurbished phone, arguing it would be safer than opening the device and replacing the battery. I'm from Portugal, so there's no official Apple stores, only 3rd party repair shops, and thus it made a lot of sense that Apple had sent a refurbished device from Ireland, than have the 3rd party shop wait for a new battery and have the effort to open the device and replace it.

However, as the 6S is not my main phone, but a spare phone, I've been using it sporadically for video capturing, as it works better with the OSMO Mobile, due to not having optical stabilisation fighting with the gimbal, and allows me to capture simultaneously with the 6S and the X.

Recently my wife's SE got much worse with the battery life - from time to time I see the battery level going down 20 to 30 points, one per second or two, and then stop, albeit the system tells me the battery quality is still above 90%. So I convinced her to switch to the 6S.

Unfortunately I then found out that the bottom microphone is unusable. The device has three microphones (or maybe 4), with the bottom one near the back cameras, and the front one near the ear speaker, working perfectly, whilst the bottom one on the left side of the lightning connector does register sound, but quite low. Using an app like FilmicPro, which allows me to pick a specific microphone, all three microphones register something if e.g. I say something, or gently blow *near* the microphones (so I'm sure which one is picking the sound), but if I hold the device and just speak, even the back microphone, behind the phone, will easily pick up my spoken words, whilst the bottom one will barely register anything.

So the device works for capturing video (will use the back microphone) and kind of works for voice calls (it uses all three microphones, for noise canceling), but when using a normal sound app, like the voice recorder, or the Sleep app that uses the microphone instead of movement, the device is unusable.

My question is for anyone with an 6S, or a 7, or a plus variation, that have suffered from a similar problem:

• did it happen due to a iOS software update? If so, did it ever get better or worse with subsequent updates?
• did it happen after a repair? I've seen that there's a possibility that when replacing the battery, the bottom board with the lightning port and the micro and the 3.5mm jack may be badly assembled and somehow affect the embedded microphone, even if the speaker, jack, and lightning still works.
• did you take your device to Apple, and what did Apple tell you? Or if you took it to a 3rd party shop, what did they tell you?

I love the 6S, but I'm not sure if I want to take the device to the local repair shop and have them charge me a lot of money to get the board replaced.

Suggestions?