Check that you've not accidentally activated the Mute function - which is controlled either by the switch that's next to the volume control or by a soft control in the multi-task bar. (To open the multi-task bar, double press the Home button, then swipe the bar that opens up to the right - the soft switch is at the far left). Whether mute is controlled by the hard or soft switch is settable in the 'Settings' menu.
Tim