Go to Settings>Sounds> and make sure the volume is turned up there. If you are looking at the home screen, tap on the volume buttons and see if it shows the word Ringer at the top of the volume bar. If it says Headphones, then the device would appear to be stuck in headphone mode. That would require a trip to the Apple Store Genius Bar. The only other thing to check is in Settings>General>Accessibility>Assistive Touch. Activate that, press the button, and then check Device. There should be two icons, separated by an arrow. One says Volume Up, the other is a bell with Mute under it. If you see anything but Mute under the bell, tap on it to put a line through it, and it should show Mute. If it doesn't that could explain the lack of sounds.