What have you currently got Settings > General > Use Side Switch To set to - and depending upon what it is set to, is the switch on the side of your iPad above the volume control on/off or is the icon in control centre on/off ?
Depending on what Settings > General > Use Side Switch To is set to (mute or rotation lock), then you can mute/unmute notifications by the switch above the volume switch, or via control centre : swipe up from the bottom edge of the screen and it's the right-most of the 5 icons in the middle of it (if the icon is white then it's 'on', tap it to turn it grey and 'off'). The function that isn't on the side switch is set via control centre instead : http://support.apple.com/kb/HT4085
Check your settings. The iPads have a small switch on the right edge. It can be used as a rotation lock to keep the screen from automatically reorienting itself as you move around, but you need to have the tablet’s settings configured properly. That same switch, right above the volume buttons, can also be set to function instead as a mute button to silence certain types of audio.
If the switch is set to work as a mute button, you can change its purpose to “screen-rotation lock” by tapping the Settings icon on the home screen. On the Settings screen, tap General on the left side, and on the right side of the screen flick down to “Use Side Switch to.” Tap to select Lock Rotation or Mute to set the button’s function. Even if you set the side switch for your preferred use, you can still mute the Mini or lock the screen. Just double-click the Home button, and when the panel of apps appears along the bottom edge of the screen, flick the row from left to right with your finger. Tap the icon on the far left side of the row to either lock the iPad’s screen or mute the iPad’s alerts, notifications and sound effects. Music, podcasts and video are not muted unless you turn the volume all the way down.
iPhone: Can't hear through the receiver or speakers
To solve some sound problems, just follow these simple steps
1. Go to Settings
2. Tap on General
3. Tap on Reset
4. Tap on Reset All Settings