What do you mean under your wall, like inside your wall? Be more specific , usually the cable box would be near the bottom of the wall and you would have a cabinet under the TV to hide all the cables since the T.V should be mounted so the center of the screen is at eye level when seated.

There are brackets made just for holding the cable box on the wall behind the TV, where the IR beam can still bounce and hit it. Can't recall the name, but just do a search on the Web.