Whatever its cinematic merits, Cadillac Records, the new film about the legendary blues and R&B label Chess, has already performed the valuable public service of exposing some of the greatest American songs to millions who might not otherwise have heard them. There would be no rock & roll without Bo Diddley's walloping backbeat and Chuck Berry's pealing guitar solos; no strutting rappers without the badass blues boasts of Muddy Waters and Howlin' Wolf; no Beyoncé crooning "If I Were a Boy" in 2008 without Etta James' pained pleading of "Trust in Me" 47 years earlier. Even if you've never heard the originals, you know this music in your bones. There's no improving on the Chess catalog, and this new compilation is a great place to start soaking it in. It includes signature songs from Diddley ("I'm a Man"), Waters ("Forty Days and Forty Nights"), Wolf ("Smokestack Lightnin' ") and James ("At Last"), as well as three tunes by Little Walter, a hurricane disguised as singer and harmonica player. The only thing you lose is how startlingly innovative these songs were in the Fifties. So concentrate instead on the still-shocking power of Waters' and James' singing, and the poetry (not too strong a word) of Berry's road songs — the most compact, artfully artless songwriting in rock & roll history. As for the soundtrack, it's an admirable curio, with performances falling into roughly three categories: valiant effort (Mos Def's Berry impersonations), slightly more polished valiant effort (Jeffrey Wright as Waters) and Beyoncé, in which today's leading diva approximates Etta James' tone and timbre without quite channeling the suffering at the heart of James' music. The album ends with a blues-rap experiment by Nas and his father, Olu Dara. It's a bit of a mess, but you can't argue with Nas' sense of history: "Muddy Waters, Howlin' Wolf, you don't stop!"