Mooie, I have the same question. We striped the ugly tile off the fireplace, and found 4 square holes; I assume they have some function. What do the side vents do? Can I brick or tile over them without effecting the functioning of the fireplace? If we use an electric insert, will it matter?

I wouldn't mess with them before getting a professional opinion. If you haven't had your chimney cleaned lately, it might be a good idea to hire a competent chimney cleaner to do that and to ask him about the vents and whether you can close them.