What you want to do, is disable all apps that are using the microphone, in order to make this happen, you'd have to go to settings, then you click Privacy, that's where you'll find Permissions Manager, click on Permissions Manager. This will take you to the next step, You'll want to scroll down now, and find Microphone. After you click Microphone, it will show you which apps, are using, or have commandeered, your microphone, preventing you from using it. Go down the list and disable everything. After that restart your phone. You'll find the problem is gone. I had discovered this problem when I couldn't use my OK GOOGLE ASSISTANT APP. Problem solved by my brilliant mind. Try it. You won't know what app it was, after disabling one app at a time, because you'd have to Restart afterwards by each one. To restart your phone, you want to press the power button. Then it will say, power down, or shut down. Or restart, and safe mode click restart for the final step. Afterwards you'll find the problem is gone. So anyways that's the solution. I know this will help as it worked out for me. Hopefully I have composed this, in a way that you can clearly understand, what I intended, for you to take away from it's crazy, composed message. Go at it now. Take care, and God Bless You.