mine has done that before, and then come back after a while randomly. You could try turning the phone on and off or managing the sound preferences, but don't worry it will probably come back. This happened to me several times and its come back...and you don't even want to know what my phone has been through, lol.
which iphone do you have? if its an iphone 4 make sure the silent button on the side is switched off. otherwise I would recommend making sure your OS is updated to the newest version...as some newer games require it..