@yayanfirli first make sure that it is turned on. Go to Settings > Sound & vibration > Audio settings > Microphone to make sure. Next check your microphone by placing your phone in in safe mode. If it does work in safe mode then the issue is a bad 3rd party App. To put an Oppo A12 into Safe Mode, you can:
# Press and hold the power button
# Select Reboot to safe mode
# Select OK to confirm
If your Mic doesn't work in safe mode either, replace it. The mic is part of charging board. Since iFixit does not yet have a specific guide for this, Use something like [https://www.youtube.com/watch?v=CTGSj4oUczs|this video] to work on our phone
While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has a device with the same issues as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.
Repair is War on Entropy!