Since updating to iOS 6 Sept 19th I have had continuous problems with no sound. If I turned the phone off (which it didn't stay off, it would automatically power itself back on in seconds), put it on the charger, or sometimes doing nothing I'd lose my notification sounds for up to an hour at a time. No text tones, alert tones, keyboard clicks, ect. Restored it. Did a master reset. I did everything. Even tried the "dock" thing. Nothing worked.
So, I went into an apple store yesterday. Initially they considered it a "software" issue and tried to do a "clean restore". Brought the phone back to me and guess what. No sound. The up/down volume buttons on the side of the phone didn't work they only displayed the little bell (no volume bar). At this point he said that it was a "speaker" issue. So the first option for this was to swap out the phone under warranty. I have my replacement iPhone 4s and it works perfectly fine. It shuts off, sound works with no problems.
I advise anyone with this problem take their phone into apple or send it in to get fixed. I don't know why, or what happened to my device but it happened AFTER I updated to iOS 6. Apple of course said that they were having these speaker problems before iOS 6 update rolled out but I stand firm---my phone was fine before the update.
Hope this helps some of you.