What seems to be an unlikely pairing of former Led Zeppelin vocalist Robert Plant and bluegrass superstar Alison Krauss is actually one of the most effortless-sounding duos in modern popular music. These two voices meld together seamlessly; they will not be swallowed even when the production is bigger than the song. They don't soar, they don't roar, they simply sing songs that offer different shades of meaning as a result of this welcome collaboration.