iOS only can set volume by physical key, CAN NOT modify by javasript

https://developer.apple.com/library/safari/documentation/AudioVideo/Conceptual/Using_HTML5_Audio_Video/Device-SpecificConsiderations/Device-SpecificConsiderations.html#//apple_ref/doc/uid/TP40009523-CH5-SW1

Volume Control in JavaScript

On the desktop, you can set and read the volume property of an or element. This allows you to set the element’s audio volume relative to the computer’s current volume setting. A value of 1 plays sound at the normal level. A value of 0 silences the audio. Values between 0 and 1 attenuate the audio.

This volume adjustment can be useful, because it allows the user to mute a game, for example, while still listening to music on the computer.

On iOS devices, the audio level is always under the user’s physical control. The volume property is not settable in JavaScript. Reading the volume property always returns 1.