How to fill a hole in the living room?
There are two solutions that should be easier and cheaper than cement or mortar.

Cement could certainly work, if you can get it to stay in place while it dries.  Get some steel wool, which mice won't chew through, and a can of spray foam such as Great Stuff (there's even a pest repellent version of it available at Home Depot).  Stuff steel wool around the hole and fill it in with spray foam, which will expand and can be trimmed with a knife after it dries.  This will keep bugs out and the steel wool will keep mice from chewing through, and it's fast, easy and cheap.  

Alternately you could get a small piece of sheet metal.   Use some construction adhesive to hold it in place and it should keep bugs and mice out.