To monitor silently, turn off the speakers if they are connected. This will also stop any echo you might be hearing. If you enable both video and audio, then you will be able to record and monitor both.

To enable noise detection:
1. On the Tools menu, click on Options.
2. Select Noise Detection from under the Monitoring tab.
3. Please make sure that the Enable Noise Detection option is checked. You can also configure the Noise Detection Delay and the Sensitivity.