Trunk-mounted sound systems are often about bragging rights, but VW might have more reason to boast than usual. It’s showing a Golf GTI Aurora concept car whose centerpiece is a hologram-controlled audio system accessible from the back. You can push floating buttons, grab 3D sliders and, of course, produce eye-catching visuals. VW hasn’t outlined the exact technology at work, but you don’t need glasses, gloves or other extras to operate it.