Hey there, mennatullah144.
Welcome to the Apple Support Communities! Thanks for the post about the audio of video recordings on your iPhone having an undesirable audio characteristic. We'd be glad to see how we can help you out.
When you are recording these videos using the different apps, are they all using the same camera, or are you using differing camera. For example, if you take videos on Instagram or Facebook where the audio is working the whole time, are you using the same camera (front-facing/back-facing) as you do when using the Camera app or WhatsApp where the audio comes and goes, or are they different cameras? Also, if you they are the same, and you test out the other camera, do you get the same result? Check out If the microphones on your iPhone aren't working, as it has some pointed steps that may help, such as testing without any sort of case applied to your device and several others.
You might also make sure that your iPhone is fully updated to the latest version of iOS to which it is compatible. - Update your iPhone, iPad, or iPod touch
Let us know what you find out.
Take care.