Volume is read-only in mobile safari

[edrpls]

On iOS this.player.volume will always return 1, we should rely on the muted property to validate the volume icons to show and how to toggle mute.

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.

https://developer.apple.com/library/archive/documentation/AudioVideo/Conceptual/Using_HTML5_Audio_Video/Device-SpecificConsiderations/Device-SpecificConsiderations.html