Cisco VoIP - How to disable the lower your voice warning on a Cisco phone
By default, the Cisco 8851 telephone will alert you when it thinks you are talking too loud. To stop these alerts, please follow the steps below.

Step 1: Press the Applications button 

Step 2: Navigate Settings > Lower your voice. You will need to scroll toward the bottom of options.
Step 3 Press Off.
Step 4 Press Ok on the pop-up window.