Just a thought:
The walls around fire places usually aren't sealed, that is, there is normally a gap between the back side of the wall framing studs and the chimney. That usually means you can fish left / right across the wall without having to actually go through the studs. You may or may not have to go up to the the top of the mantle to go across but its better then going down and measuring for the wall cavity to drill up.
I don't have any pictures but my parents mounted their TV above the fireplace and I bought them a HTIB system for them a few years ago. We just cut in a new work box behind the TV and a new work box down behind the AV stand to the right, normal outlet height off the floor. We were able to fish between the two cuts outs without issue.