No video sound when button silent/sound (above the volume buttons) is activated, is it a bug?

There's a bit of me that thinks that activating the switch ought to kill all output from the speakers under any conditions, however if I try to play a video on my 6+ iOS 9.1 the switch has no effect. Try resetting the device, or explore the settings. I couldn't find any hints that the behaviour is optional. 😕

tt2