Only a fool would mess with the formula that led to Mumford & Sons' junior-Coldplay coronation last year - and all signs indicate that the British alt-folk sensations have no intention of doing so on the follow-up to their sleeper smash debut LP, Sigh No More. Take their latest appetizer, a dolorous ballad that they previewed at a recent radio performance. "Ghosts That We Knew" (or maybe just "Ghosts") opens with soft acoustic fingerpicking and a typically careworn vocal from frontman Marcus Mumford: "You saw my pain washed out in the rain," he murmurs, bruised but still wanting to believe. His bandmates join in with ragged harmonies on the chorus - "I will hope as long as you like," Mumford pleads, "just promise me we'll be all right" - and their twangy contributions on banjo and accordion further the Appalachia-via-London catharsis. When Mumford's voice strains and sputters in the final refrain, it only makes him sound more passionate.