No Sound on Any Apps

Hi - I just got an iPhone a couple of days ago. I have a 4S and have updated the software. Today, I downloaded a Fisher-Price Laugh and Learn app for my daughter. It had sound for about 30 seconds and then the sound was gone. I exited out of the app to find that I don't have any sound on any of my apps! My keyboard sounds are gone, too. I deleted the app and still no sound on anything else.

I have checked all of the obvious. Does anyone know how to fix this?

ETA: Ok, I have just discovered that all my sound is gone. It doesn't ring either.

TIA