After you set Shade smooth by angle either press F9 or expand the adjustment box in the bottom left corner of the 3D viewport. You can set the angle there. The recommended method is to use the modifier instead of this. To do that click add modifier->normal->shade smooth(sorry I don't remember the name) in the modifiers tab of the properties editor.
I tried the first method and I was able to set the angle. Everything looks fine except the speakers. How can I fix that?
