*smells discussion about esoteric evocation elements, bursts in*
My favorite topic! I'd put Wood down as either part of Earth or another element entirely, like in Wu Xing. Earth is the obvious over-element for Metal, but Fire has a solid case too as both of them are symbolic of human progress and technology. Sound is definitely a subset of aeromancy (Air magic). Spirit is mental stuff, light, sometimes sound, veils, illusions, and force stuff.