Sound management in iOS is a complete debacle for Apple.
Like you say, there is no way to effectively and reliably make an iOS device shut up
. The new pseudo-feature introduced in iOS 6, "Do Not Disturb", does not
guarantee you will not be disturbed. Several alarm apps are not affected by it.
It's a really embarrassing situation for Apple. For heaven's sake, there's a hardware button
on the iPhone for switching off sounds, and even that doesn't work reliably?
They should give those of us who need it, the option to push the hardware button, which would guarantee
that the phone would remain silent under all circumstances. This currently isn't possible.