Mamma mia, what’s this? Freddie Mercury, the man who perpetrated heavy-metal opera in ”Bohemian Rhapsody,” shrieking alongside an actual opera star? In 1987 Mercury somehow conned a world-class diva, Montserrat Caballé, to slum with him for an entire album. The result apparently was deemed too flaky to be released in the U.S., but in the wake of Queen’s commercial resurgence (and Mercury’s death last November from AIDS), any curio is up for grabs. The title track has even landed on an all-star pop salute to the Summer Olympics. And why not? All the material in Barcelona (cowritten by Mercury) is penned in a style meant to snub rock in favor of ”real” opera, but likable pop hooks keep peeking through. The stuff lands somewhere between Andrew Lloyd Webber and ”Climb Ev’ry Mountain.” Such a goofy context ultimately makes Caballe seem less like Maria Callas and more like Yma Sumac. But then, Mercury’s reverence for the star and his flair for kitsch make this a novelty item too cracked to resist.