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When creating custom interfaces around the HTML5 <audio> and <video> elements, make sure you hide the volume control if the user is on iOS. According to Apple's documentation, the volume property of <audio> and <video> elements is read-only, and always returns 1:

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.

— iOS-Specific Considerations for Safari HTML5 audio and video, under the "Volume Control in JavaScript" section.