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 :)
My character isn't positioned at the center of the camera, but rather at the bottom, but I want sound to be listened to from that point. Hence the ability to set an arbitrary object with x, y etc. Camera also does not have velocity, but a sprite might have which gives more realistic doppler effects.