Kevin Spacey’s vocals were by far the most impressive thing in his bizarre Bobby Darin bio-pic “Beyond the Sea,” which quickly disappeared into oblivion a few seasons ago. Spacey even toured as a singer to promote the flick. Now Spacey, who has been making noise about retiring from filmmaking, is teaming up with Dean Martin — yes, the late singer — for an album. He’s dueting with old tracks by the dead Rack Packer on two numbers, “Ain’t That a Kick in the Head” and “King of the Road.” Other performers who will be heard on “Dean Martin: Forever Cool,” to be released on Aug. 14, include Robbie Williams and Joss Stone.