When muted, restore the volume to the one previous to the last mute call.
When the volume is already superior to 0, this call won't do anything.
As the volume is not dependent on a single content (it is persistent), this method can also be called when no content is playing.
player.unMute();// mute the current volume
player.mute();
// unmute and restore the previous volume
player.unMute();