Room is way too bright.

EQ. 

verticle blinds can be had using softer fabric. Also, a much better solution, I did one room where a largest window behind the listener was fitted with plywood covered with glued on acoustic foam panels. Looked good and worked great and was easily removable if need be.

Try blinds from Hunter Douglas, totally worth it if you value an elegant and long term solution. My room has 3 windows (fortunately) and I went with Duette Cellular shades. 

https://www.hunterdouglas.com/window-treatments/sound-absorption

If you can't do anything else there's a variety of window film and installers.  Personally I dislike traditional blinds as I find they often don't cut enough light, or leave obnoxious holes of light through.

If you need to cut solar heat transfer you'll want metalized films, but if all you want is darker you can definitely do that with a variety of film with different grades of darkness.

Of course, there are more advanced slide up/slide down blinds as well.

Vertical Blinds, Thick Acoustic Fabric, angled one way facing the speakers both absorb sound and block sound reaching glass/hard/reflective surface behind, and still maintain some daylight and view from other vantage points.

Heavy duty tracks, pro install for the weight, several separate sections to be individually angled/open

Consider the whole wall, above and below the windows if not full height.

What is opposite wall? Even though no windows, you could have similar to control side reflections, pull open to reveal artwork, storage, equipment on wall when not in use,