Pics would be nice. When I opened up our fire "box" it was just that, a stud wall area surrounding the fire box. There was nothing structural about it. Then again, I live on a slab constructed town house, a trussed roof.
I ran 2.5 " PVC for wire chases. One to each side, and one up to the attic. If you use PVC, make sure all your cuts are square for the glued joints. Mine weren't, and when I went to pull wire, it would get caught up at the joints where there was a sloppy cut that left a rough area inside the turns. HTH.
You can read more in the sticky, Putting a TV above a fireplace.