I won't comment on the audio part, there are just too many options, but if as you say, the room is naturally dark, and as big as it is, I think a projector and a large retractable screen would be way better.
If it was a barn, I assume it has a pitched (cathedral) ceiling with the high point in the middle running the long dimension. The slickest way to do the screen over the FP, would be to construct a level soffit about 9 feet above the floor and projecting about 3 feet out from the fireplace end. That would provide a natural place to install a motorized screen so you wouldn't have to look at the damn thing when you're not using it. As for flat or other type TVs over fireplaces, it just looks wrong! Whether the TV's on or off, its presence competes with the fireplace and you're not sure what to look at, the fireplace, or the TV screen! I always advise my clients against it.
If it was a barn, I assume it has a pitched (cathedral) ceiling with the high point in the middle running the long dimension. The slickest way to do the screen over the FP, would be to construct a level soffit about 9 feet above the floor and projecting about 3 feet out from the fireplace end. That would provide a natural place to install a motorized screen so you wouldn't have to look at the damn thing when you're not using it. As for flat or other type TVs over fireplaces, it just looks wrong! Whether the TV's on or off, its presence competes with the fireplace and you're not sure what to look at, the fireplace, or the TV screen! I always advise my clients against it.