If you are experiencing issues with the phone's microphone, try using the solutions below to fix the problem.
A common issue for this might be that the Mute button is on. Check that the microphone is not muted.
Test the microphone by connecting a wired headphone that has a microphone. If the headphones work while connected, remove the headphones and continue onto troubleshooting the internal microphone.
Test the audio by going to a quiet area and restarting the phone. Once restarted, record a new voice note by going to Messages and attaching a a recording to the message. You may also try and do a Skype Test Call. Skype is available from the Windows Store to download free of charge.
Finally, the issue may be a simpler one. Check and see if the microphone's opening is blocked. If so, use a dry cleaning cloth to wipe out any dirt or blockage.
Again, if neither of the above works, you may simply need to replace the microphone.