The symtoms you discribe do indeed sound like the mute slider switch, or its muted, as that's exactly the effect you get. some operation sounds work but not the main sound, but you say you have checked this.
As you can hear some sounds this rules out there being a problem with the earphone jack. It certainly sounds like you have a setting somewhere muting it.
Make sure bluetooth too is disabled in case somethings got screwed up there.
I doubt it will make much difference in this instance, but when desperate, if your've not already done it. Always worth doing a none destrucive reboot, by hold down the Home and on/off buttons at the same time... you never know.. Hope you sort it out... let us know what it was.