What about when you pull the notifications down? Tap the sound toggle see if that changes anything.
*** Mays galaxy ****
I also have the same problem. I called Samsung themselves to see if they could help me fix the issue. They guy told me that it sounds like a software problem. I called my phone company since I have only had my phone less then a week. They replaced it with another brand new phone. Guess what?! It happened again!!! So if anyone has there issues fixed please post so I too can have my issues fixed...thanks for any help someone can give!!
I had the same problem and fixed it. Sort of....
Two things to look at are what you use to change the volume, and recent apps, as others have now stated. A widget could be getting "caught," so to speak. If you only use the volume buttons or the sliding bar in the notifications I don't think it applies. Emphasis on think. For me it was an app that took over the sound. Like if I played that game and listened to music at the same time, you could only hear the game. It would also only happen if the phone was on silent and I opened up that game, for whatever reason it seemed to keep it like that.
So some things to do are when/if it does happen again, hold the home button and see what apps are currently running. Go through one by one and close them and see if it changes. Most likely it will and you'll have eliminated it down to the source of the problem. Now check settings in that app to see if anything controls the sound etc etc. Play around with this, see if it helps...
So I pretty much force closed that app
For what it's worth I've experienced this same problem on my Bionic and once sofar on my SG3. Its never happened often enough for me to spend time to chase down the cause. Interesting discussion.