If you use a non-original cover that covers the holes for the microphone, this may be the reason for the poor sound on the videos you record. It may also be because dust and dirt accumulate in the speaker and microphone holes over time. Check whether there is free access to the speaker at the top and the microphone at the bottom. Since the sound is not loud during calls, and since the sound in the videos you record sounds wrong, the above could be one of the reasons.
Also remember that during calls, when you have your iPhone to your ear, to use the Volume buttons on the side of your iPhone to adjust the volume.
There is also the possibility that either the speaker or the microphone may be defective.
You can find more useful information here:
If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker - Apple Support