Can you be a little more specific? Middle of the walls? Toward the corners? Try to keep them away from front corners of the room and the window away from your reflection points.
In general, you'll want some broadband bass control in the front corners and reflection panels on the sides at a minimum. Depending on speaker type and location to the boundaries, etc. nothing, absorption, or diffusion behind the speakers may be in order.
Rear wall - again, depends on how everything else falls whether you'll need bass control in the middle of that to deal with bass nulls.
Diffusion on the rear half of the side walls is always a nice thing if budget will allow.
Lead Acoustical Designer
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