Issues with sound are often caused by the side switch settings which can control either lock rotation or mute functions on your device. Please read full details on Apple's website: https://support.apple.com/en-us/HT204171
In Settings --> General --> Use side switch to:
If the side switch control settings are set to mute, when the toggle is switched so that orange is showing, you may not have sound coming out of your device, even if the volume is turned all the way up. Simply flip the switch (so no orange is showing), and check that the volume is turned all the way up. This should resolve the issue.
If the side switch control settings are set to lock rotation and you still have no sound, slide up from the bottom of your device, and look at the last icon. It will control sound. Make sure it is NOT highlighted (In the image shown below, the audio functions are highlighted. Tap it to un-highlight it).
If adjusting these settings does not restore the app's audio features, please e-mail email@example.com with your device model (eg. iPad 2, iPad Air, iPad mini with Retina Display) and your operating system (eg. iOS 7, iOS 8). For more details on the side switch settings, lock rotation, and mute controls, please read more on Apple's website:https://support.apple.com/en-us/HT204171