Volumetric materials do not apply to metaballs, terrains or lattice. I wish they did. And if I'm wrong, please tell me how. Depending on the density of the fireballs, making many into one could help. It mostly depends on the detail you'll lose. I like the way you've wrapped fire around the falling stones. The problem I find is the overlapping edges of the smoke spheres are interacting with one another and you are getting some sharp edges. The fire spheres bland pretty well.