Hi and welcome to the Android Forums!
Boy that could be a number of things. Have you tried a simple battery pull? If that doesn't work, I know this sounds silly but go into Settings > Volume and make sure the volume is up. It's possible that you have a bad volume rocker and that will confirm that the sound is "up."
If those things don't work I suggest you take the phone in to your carrier and let them have a look at it.
Good luck, I hope that helps.