Add basic positional audio to the sound system.

Use: game.sound.listener = someSprite; to set the listening position to follow that sprite. (Usually your player character). And then use someSound.following = otherSprite; to make the sound follow that sprite. set following to false to stop the sound following a sprite.

Even if this is not the way to go, it might serve as an example on how to approach this :)
Probably just want to do a falsey check here; people will likely assign `null` to this sometimes as well.