Hi,
Have you checked that the microphone 'input hole' is not blocked, either by a phone cover, dust or lint.
Use a strong light and a magnifying glass to inspect the 'hole' which is located on the bottom edge of the phone,
DO NOT use a pin or similar object to clear the hole if it is blocked, you may damage the microphone which is right behind it.
Here is a link to the Ifixit guide for your phone. Using this will help you to gain access to the hole to clear it if it is blocked.
If you do not wish to open the phone you could try using a vacuum cleaner to see if it will suck out the blockage.
As to why the recording videos and speaker mode works I'm not sure. Perhaps they use a different microphone or maybe they use the 'loudspeaker' as a 'microphone' when in this mode. Speakers can work in reverse to act like microphones.
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