Wouldn't you want your speakers mounted flush with your TV? Squeezing anything between the window and fireplace puts the speaker right next to 2 hard acoustic surfaces, creating unwanted reflections.
Not knowing the quality of sound you are expecting, theater or stereo sound, or a budget, it is hard to make any specific suggestions. Three little satellite speakers mounted alongside and just below the TV works for lot of people as do sound bars, either passive with external receiver or active, connected directly to the TV. If you are thinking of something more invisible, there are many directional in-ceiling mounted speakers that will let you angle the drivers towards the listening position. Look at Triad, Sonance, or Speakercraft for examples.
Don't see a lot of evidence of wiring pathways for video or audio on that fireplace or surrounding wall; how finished is this space currently? You would think that there would be quite a bit of wood framing around those two windows, not leaving a lot of space between them and fireplace for routing wire. Is there a basement below?