By way of Club Nintendo
Nintendo has a pretty cool promotion going on for all you Smash Bros. fans out there. If you buy both versions of the upcoming game on the 3DS and Wii U, you'll get a free soundtrack. It looks like there are two discs, red and blue, which likely refer to the 3DS and Wii U versions respectively.
All you have to do is register both games on Club Nintendo to get the deal. Although it seems as if it is a physical soundtrack offer (as Club Nintendo has done in the past), Nintendo says "more details are on the way," so nothing is set in stone.