Microphone in Camera app doesn’t work fine

Thank you for checking that out, sorry the problem remains.

Here';s another article that may help:

If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker - Apple Support

Also:

Wondering which iPhone X model you have.

If you have the option, does the camera have stereo or mono sound enabled ? ... maybe compare them ?

Change the camera’s video recording settings on iPhone – Apple Support

Says:

Turn stereo recording on and off

On iPhone XS, iPhone XR, and later, iPhone uses multiple microphones to achieve stereo sound.

To turn off stereo recording:

go to Settings  > Camera, then turn off Record Stereo Sound

Something else you might want to check:

If your iPhone is stuck in headphone mode - Apple Support

Thank you for your reply, simajonas.

Sorry that I don't know what causes the sound issues to be there at times, and not at others.

To rule out, reduce, or overcome possible issues caused by wireless interference, see Apple support's article below:

Resolve Wi-Fi and Bluetooth issues caused by wireless interference - Apple Support

Re: Is it possible, that dust is in there and this makes the crackling sound?

See: About splash, water, and dust resistance of iPhone 7 and later - Apple Support

As the microphone isn;'t working as expected

Try this Apple support article:

If the microphones on your iPhone aren't working - Apple Support