I am not much sure how to mute the keypad tone in Samsung Corby unless you select the profile to Silent. But changing the keypad tone to Beep will not make much noise when you are in office where you may not need the phone to make noise while you use it.
Here are the steps how to change the keypad tone to Beep, Select Settings->Phone profiles->Select the profile-> example Normal or Office -> Select the arrow button (>>) Select the drop down list which has options of Incoming, New Messages and Phone sounds. Select Phone sounds and select the drop down list for keypad tone and select Beep and save. By doing this you would see the phone doesn’t make much noise while you navigate the menu or type a message.