If you press the "Sleep/Wake" button when your iPhone is open to the Home page or to an application interface, the device will lock, rendering the touch screen unusable until you activate the unlock prompt. By default, pressing the "Sleep/Wake" button to lock the iPhone produces the lock screen sound effect. Disable this feature if you don't want the iPhone to produce the sound effect when you lock the screen.
Tap the "Settings" icon on your home screen.
Select "Sounds" from the Settings page. The Sounds preferences page loads.
Scroll down the page to the "Lock Sounds" field. Slide the switch next to the field to the "Off" position.
Tap the "Settings" arrow at the left of the top toolbar to return to the main Settings menu.
Press the "Home" button to to your home screen.