Sound suppressors are regulated under the same regulation that sees over transfer of full auto toys. It's perfectly legal under Federal Law within the regulations of the National Firearms Act of 1934. The thing that puts it into a FUBAR situation is the states when they start to inact some goofy laws, I think it's Washington where it's legal to own a sound suppressor but it's illegal to actually use it/fire it. Then there's California which makes a habit out of denying any requests to own NFA items.
To put one on an AR15, the thing needs to be a pre-94 Assault Weapons ban rifle that is entitled to all it's evil little features. Sound suppressors are infact VERY effective flash suppressors since they contain so much of the muzzle blast and cool it so effectively to suppress the muzzle blast, with post94 AR15s one of the things they can't have is a flash suppressor so therefore no sound suppressors for post94 "neutered" AR15s.