Don't feel alone, I have the same problem sometimes. For example; yesterday I got a text message and it audibly informed me, a few text messages later I noticed that the screen would light up and tell me I got a text and even vibrate but no sound? It's a little flaky! I am going to post feedback to Apple with this problem.
So the new update has been out and I assume most everyone has updated. I have updated and still have the alert sound issues. Anyone else have comments? 😕😕
Well, doing a restore solves this issue for the time being, but the problem always come back. I don't live close to an apple store so I have a feeling when I go, they will reset it and it will work for them right then.