They will obviously reflect sound differently than bare windows. If that is a good or bad thing will depend case by case. Its the acoustics of the room as a whole that matters, not any single part, though any of those might be tweaked as needed to get desired results.
Please comment on your experience with Plantation shutters. Yes they look great,
but do they act as good sound diffusers over windows? Has anyone noticed they improved the sound in a room with windows? I am thinking they may do a good job when tilted, but not fully closed. Perhaps even better than 2 inch wide standard shades?