You could follow the steps below to enable/disable audio for recording and live viewing on your NVR.
Applies to: RLN8-410 (hardware version H3MB18/N2MB02/N3MB01/N7MB01), RLN16-410 (hardware version H3MB18/N6MB01), RLN36, NVS36 and RLN12W, NVS12W.
Here is a video for your reference.
Step 1. Log in your NVR on a monitor, then go to Setting > Camera > Audio and Light. Choose the channel that you want and turn on the Record Audio button.
Step 2. Left click live view image of the camera to pop up the toolbar, click the Speaker icon to activate audio for this camera.