IF YOUR IPHONE THINKS IT'S ALWAYS DOCKED AND THE VOLUME SLIDER DISAPPEARED WITH NO AUDIO OUTPUT you need: Jailbreak IFile [Hidden Content] Only try this method if you already do this:
Make sure the Ring/Silent switch is not switched to silent.
Check the receiver mesh (which is on the top front of the device, above the display). If it appears blocked, use a clean, small, dry, soft-bristled brush to carefully and gently brush away any debris.
If you have paired with a Bluetooth headset that is nearby, either use the headset or turn Bluetooth off (chooseSettings > General > Bluetooth).
If restarting the iPhone doesn't resolve the issue.