I have a large, dark entertainment stand with closed glass end/doors. I cut posterboard to size and used painters tape to attach it to the back. You could also staple fabric or paint foamcore board. I painted my posterboard the same as my wall color (BM Shaker Beige) and now things don't disappear.

Since the doors are pretty formal, I'd keep the styling simple. You don't want both the doors and what's inside fighting for attention. I'd use books for the bottom and stick with larger items for the rest. Heaviest things on the bottom. I'd only put one item on the top of each sides.

Thanks @sheeisback ! I was thinking of doing exactly that Do you by chance have a pic of yours?

How would this look without the doors?

Can you take the doors off? Otherwise, you need to do as others suggested: either put a solid cover behind the glass, or only put books or some other solid visual on the shelves behind the glass. Too many small things doesn't look great.

I think the doors are the prettiest part and would hate to see them removed. I would, however, limit the display pieces inside considerably. I think there are too many little things going on and would edit them to perhaps one large item on each shelf. For instance, I'm envisioning a collection of large white pottery pieces. Something unified but each item distinct.

I like the idea of 1 big piece on each shelf. That's easy enough for me. Any suggestions for that long, open, top shelf?

Can your two center speakers be moved to the long top shelf?

I honestly don't think it looks like it needs tons of improvements. How desperately do you want folks to be able to see what's in the cabinets? I'm sure in real life they look clearer than from the photo. The thing I think most detracts from the look are the small black speakers on top. You say they are moving, are they needed? If so, they would look better on that top shelf than on top against the light wall. If they blend into the background of the dark top shelf, so much the better. Is navy part of your color scheme? If not, I would try to find something more neutral to cover the bottom doors, maybe some type of grasscloth? Or just paint or stain the wood underneath. Can't quite figure out what is up with them.