Hi Stirling & Jill,
The sound has just stopped working on my iPhone 4s as well. The volume option has also simply disappeared from the Control Centre & Siri 'doesn't know what I mean' by Volume, Sounds or Sound Settings. The only way I can hear any tones (ring, alert, keyboard clicks etc) or music is to connect headphones.
Here's what happened: Everything was normal, I had 42% battery & was taking a photo (standard camera, not in-app camera), when my phone powered off, then when I tried to switch it on, it indicated that the battery was flat, so I plugged in the charger, it switched on immediately & said it had 39% battery level AND NO SOUND.
I have tried the above advice with no result - it's incredibly frustrating.
There is a software update available (7.1.1) but hesitate to install it as I was advised to update to IOS 7.0.4 only a few months ago to "fix" a software bug, not only did it not work, but my battery life went from 20 hours to 5 hours (which I still find appalling & unacceptable).
I would appreciate any advice you have on how or if this known fault can be fixed.
Thanks in advance.