Hello Nicola0,
To start, the steps you mention "record a memo, record a camera" are not a way to fix things. These are isolation steps to help identify if the issue is hardware based, or related to your cellular network/connection so you know how to proceed.
iPhone has three microphones on it, the main one at the bottom, a front facing one and a rear facing one. When you make a normal phone call the main mic at the bottom is used. When you place a call on speaker, the microphone that is used depends on the device orientation. If the display is facing up towards you, then the front facing mic will be used. If the device is display down with the back side facing you, then the rear facing mic is used for the call.
That's the reason for testing each individual microphone, to see if one of them has a hardware issue or not. Voice memo uses the main mic at the bottom, a video taken with the front camera uses the front facing mic and a video taken with the rear camera uses the rear facing mic.
So for example, if you normally have the device facing up when you place a call on speaker, the front facing mic is the one you'd want to test by taking a video with the front camera. If the audio is fine in the video, then the microphone is working properly and the issue is coming from the cellular network/connection and you'd want to contact your wireless carrier to report those issues.
If a video taken with the front camera also has distorted audio, then that would indicate a hardware problem with that specific microphone and you'd want to contact Apple for service options: Get Support
Cheers!