A common problem for Apple iPhone users is the microphone is missing from the keyboard. Users expect it to appear in the lower-left corner of the keyboard when using any app. There are a couple of things you can check to solve this issue.
Enable Dictation under “Settings” > “General” > “Keyboard” > “Dictation” > “On“.
Enable Siri under “Settings” > “Siri” > “On” > “Enable Siri“.
In some instances, I have seen the microphone go missing or become grayed out within specific apps. This happens frequently to me in the Google Hangouts app. Close, the app by double-tapping on the Home button, then swiping the app upward off the screen. From there, start the app again by selecting the icon on the Home screen. The microphone should then appear on the keyboard.
Sometimes, even if Siri is enabled and you’ve closed the app, you’ll still have problems with the microphone. Shut the phone off by unlocking the screen, then pressing and holding the “Power” button for about 5 seconds. From there, switch the “Slide to power off” option.