New devices are configured by default to enable the key beep sound when a key is pressed.
To remove this, exit the payment application (Manager Menu->Exit) into Program Manager.
In Program Manager, press 0 then 4 for Sys Settings
At the top of the list is Key Sound. Press the green key and use the up/down arrow keys to change this to N
Press red twice to go back to Program Manager and press the green key to restart the payment application.