There's already an optical out directly above the coaxial out. gonk's guess sounds logical. I don't know what the whole section is for. The spacing just seems...odd. There are 2 pairs of red and white connections in with that mystery plug. One pair above and one below it.

Take a look at the small hole in the space next to the white square. Could that one be a USB port? It might be more the right size. There is something small with a label there.