The dark accent wall doesn't make much sense, unless you make it a feature of the room. You could put the sofa there with two end tables and art above them to create some balance and make the room more inviting.

I'd put the piano centered on the dark wall! That can act as your "fireplace" or focal point. Put the couch opposite, facing the piano. Use one of the end tables with the chair and the other with the couch and put the bench under one of the side windows. I'd lose the big coffee table and find a low round one. It will work better for flow and people can sit on the floor by it and listen to the music.

You can arrange wall to wall sectional near the window and also use white rug to balance the lighting of the. use off white color shade for sectional. that good match with your wall color. for sectional idea check out the ideabooks http://www.houzz.com/ideabooks/10881285/thumbs/Living-Rooms