Hi elidrissi,
We understand your microphone isn't working in WhatsApp. We're happy to offer some guidance.
If your microphone is not available or not picking up any sound in that application, you'll want to make sure you have access allowed for it. You can check in Settings > Privacy > Microphone, to make sure it is turned on for that application.
You'll also want to test out each of the microphones to make sure they're all working and the issue isn't hardware related.
iPhone has 3 microphones, one on the front, one on the bottom, and one on the back. You'll want to test each of these to make sure they're all working.
To test the front microphone: Go in to the Camera app, switch to the front facing camera and put it in video mode. Take a quick video of yourself talking or some sound, then play it back to see if it captured the audio.
To test the rear microphone: Go in to the Camera app, switch to the rear facing camera and put it in video mode. Take a quick video of yourself talking or some sound, then play it back to see if it captured the audio.
To test the bottom microphone: Make a phone call through the regular Phone app, and confirm that the other person is able to hear you.
If you have confirmed that all 3 microphones are working and the issue is just with that application, you can try uninstalling and reinstalling it.
Delete apps on your iPhone, iPad, and iPod touch
If the microphone still doesn't work in that application, you'll want to contact the developer of it to let them know of the current audio issues it has.
How to contact an app developer
Hope this helps. Cheers!