My iPhone 5 has stopped aural text notifications from outside parties. If I send myself a text from my iPad, it'll ding, but it's stopped notifying me when friends text me, most inconvenient. I just performed a Restore to no avail on this.
Check your "Do Not Disturb" settings? If a restore won't fix it, and all the settings are correct, sounds like a SW bug rather than the phone itself, but if it persists, take it in.
If you've updated to iOS 7, go into notification center and make sure the sound is turned on in there on your phone.
I had the same problem and that's what it turned out to be, the sound was on under the messages app, but not in Notification Center, but once I turned it on in there, all was fixed.