Currently, the ARIA value of VolumeBar tracks the value of volume. It should instead track the position of the slider on VolumeBar, which tracks volume except when the player is muted, in which case it's 0.
Fixes#4064.
If a user changed the volume to zero either via the mouse or keyboard, clicking unmute will now restore the volume back to this last position. Previously, the mute and volume values were completely not linked.
Fixes#3909.