Very not-straightforward. A compulsory mechanical license to distribute a derivative work as a sound recording does not automatically grant you copyright over the derivative arrangement of the song itself. You kind of own the sound you recorded (you pay a statutory publishing rate, which is around 10 cents, per unit sold to the songwriters, which the Harry Fox Agency collects and distributes on behalf of the music publishing industry) but you don't actually own the song.

That said, it's most certainly not nice/cool/ethical to steal non-copyrighted arrangements simply because someone can't sue you.