Hello Lecach,
First, ensure the iOS version is fully up-to-date (Settings > General > Software Update) to ensure the latest improvements/fixes are applied: Update your iPhone or iPad - Apple Support
For odd issues that come up, it may sometimes help to restart the device, if you haven't yet since noticing this: Restart your iPhone - Apple Support
1. Press and hold either volume button and the side button until the power-off slider appears.

2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, force restart your device.
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.
This page can also help to provide help for microphone-related issues: If the microphones on your iPhone aren't working - Apple Support
1. Remove any screen protectors, films, or cases from your iPhone.
2. Clear any debris from the openings of your iPhone.
Cheers.