1: The apply button is only really relevant to resolution and screen mode, the others settings are applied live. If you have set up the correct sound channels (See manual part 3.10 or part 4), you should hear the volume change when dragging the slider.
2: Fullscreen option does not work well inside editor. Package the game or run it as standalone if you want to test out fullscreen.
https://i.imgur.com/jXAwDN9.png