If you happen to install CM Locker, press the upper left button from that app then choose Notification Cleaner, from there, press the upper right button setting then turn off the notification from messages. That means message's notification will not be muted. Hope this will help.
Just to confirm with you:
Have you force stopped and cleared the cache of the messages app?
Have you cleared the cache partition of the phone?
Have you gone to settings> applications>application manager> then message, and verified that it is not set to Block?
That may have fixed your issue but mine would still only vibrate. I found a possible alternative solution. Go to Settings>Apps>messages>notifications>importance and click urgent. This will make a sound and vibrate when receiving messages.
I ultimately found the reason mine wasn't making a sound was due to moving my downloaded ringtone to the sd card. So I went into notification settings and changed the default sound to one of the provided sounds from the list.