The plastic would definitely muffle the sound somewhat but if you like the look then just put them up there and try it with your plexi in place. If it is okay then you never had a problem. If it is horrible it is time to go to plan B, whatever that is. Behind the plexi may not be worse than into the floor, into the ceiling, or into your knee which are the other viable options suggested. I agree that speaker cloth on the front of that design would not look best.