I personally would not put the TV over a fireplace like that, as it is too high, and also there is a risk of damage in the event of a fireplace mishap. Plus there is nowhere to put in a proper surround system.
But if you are going to have it there anyway, you might want to just place a pair of bookshelf speakers on the mantel, one speaker at each end of the mantel. You then would not need to bother with a center channel, as the right and left would be close enough to the TV to anchor the dialog to the TV. This is far from ideal, but the TV placement is far from ideal, so I don't see that as a good argument against it. You will probably want to place them tweeter end down so that the tweeter is closer to ear level, though it is still going to be high (unless you will be sitting on high chairs).