Click on the Server icon at the top left of the Agent UI and "Add Device" under Devices. Or in Live View click on the Edit icon at the bottom right and "Add New Device"
You can add Microphones as stand alone devices that record audio to audio files or you can pair them with cameras and the sound will be recorded into the camera recordings. To pair a microphone with a camera see Editing Cameras
Tip: If you add an IP camera that has an audio track in the stream Agent will add and configure a paired Microphone control automatically.
Click on Audio Source to add a new microphone
The second step when adding a new device is to setup some basic features. If you check "Alerts" Agent will setup your microphone with a sound detector so it's ready to raise Alert events. To record when Agent detects sound check the "Record on Detect" option. These options can be edited later.