In order for AirServer for Mac to record a voiceover via your microphone, it needs to get permission to access the microphone. To verify if you have granted permission or not, you can go the Privacy settings in macOS. You can find this in System Preferences - Security & Privacy - Privacy - Microphone.

If AirServer is not listed there, you will need to make the permission window appear in AirServer for Mac first. AirServer will automatically show this permission window once you unmute the microphone after starting a recording.