First make sure that the camera firmware is up to date. There was one firmware that did not have the Audio menu. Once the firmware is up to date log into the camera on the web browser and navigate to the audio menu. Make sure that audio is enabled and the mic is set to line in one.
If you are using the 12V on the camera for an output navigate to the maintain menu and set the 12V output option on.
Extra option is to put the mic boost to 255 db. This will make the camera louder but can boost static noise.