Videos, Games, Siri can/do make noise when "ring/silent" switch is engaged. I usually don't have a problem but sometimes Siri launches when I don't expect her to - quit an app... wait... pling-pling-pling!
Yes that's how silence works on iOS. Mute mutes thing you have no control over calls,texts etc. why the hell would I fire up a video and not want the sound?