Playing silent sounds on iOS 8?

I've been trying Sharkfood's Sound Switch for detecting whether the mute switch on an iOS device is turned on.

Their mute.caf for "silent sound" looks indeed silent (i.e. flat) in a sound editor, but when I run their code I hear it as a hissing sound on iPhone 5 (when the device is not on mute, of course).

Is this a feature or a bug, i.e. does iOS perhaps acively prevent against the playing of "silent sounds"?