The iPhone's primary form of user input is its touch screen. This touch screen can detect multiple points of simultaneous contact, allowing you to type freely using the on-screen keyboard. To provide tactile feedback when tapping on-screen keys, the iPhone makes a clicking sound when a key is pressed. There are several reasons why the iPhone's keyboard clicks may not be audible.
On the side of the iPhone next to the volume up and down buttons is a small rocker switch. This switch is used to put the iPhone into silent mode; the device only vibrates when receiving a call or performing any other kind of alert when the mute switch is active. The keyboard's clicking sounds are also muted when this switch is active. Flip the switch into its upper position to deactivate it.
If the iPhone's volume has been turned down to its minimal setting, the keyboard clicks are not audible. Adjust the handset's volume using the two buttons on the side of the casing. Hold the up button to increase the volume until you can hear the keyboard clicks again. If the volume is turned down and the mute switch is active, un-muting the iPhone does not increase the volume, which can cause confusion over whether or not the device is still muted.
Keyboard clicking sounds must be activated in the iPhone's settings, or they will not be audible. To activate keyboard clicks, launch the "Settings" application by tapping its icon on the home screen. Tap the "Sounds" button, and slide the "Keyboard Clicks" switch to "On." The iPhone must still be un-muted and have its volume turned up for the clicking to be audible.
When holding the iPhone in its landscape orientation, and typing on the on-screen keyboard using your thumbs, it's easy to accidentally cover the iPhone's speaker with the heel of your palm. The speaker is located on the bottom of the device; care should be taken when holding the iPhone that the small, oval grill on the bottom edge is unobstructed so you'll be able to hear the keyboard clicks.
If you've tried all the volume and settings options and still can't hear the keyboard clicks, it's possible that the iPhone or its operating system has developed a fault. If the iPhone still makes other sounds, such as music or a ringtone when receiving a call, it's likely to be an OS fault. Connect the iPhone to your computer using its USB cable and reinstall the OS using the "Restore" function in iTunes. If the iPhone is making no sounds at all, contact Apple support or take the device to an Apple Store for information on repair or replacement.