iPad users, with iOS 4.3, now have the option to control the functionality of the device’s side switch, pictured in the image at right. This switch allows you to control muting of all sounds or (its original intention), to lock the screen orientation in either portrait or landscape. This article will show you how to change the setting to however you prefer.
Scroll down to "Use Side Switch To:". This option will only be visible on devices running iOS 4.3 or later.
Tap to check your desired preference: Lock Rotation allows you to keep the display from changing to portrait from landscape mode, or vice versa. Mute silences all sounds on the device.