In trying to get a handle on The Umbrellas of Cherbourg, I wrote about the Herbert Ross version of Dennis Potter’s Pennies From Heaven, deciding that it spoke to the apparent impossibility of ever making another great Hollywood musical. Why wasn’t it obvious then that the Coens, with their innate eccentricities, flair for grand theater, and command of editing rhythms, would be just the folks to reinvent the genre? O Brother, Where Art Thou? only gets partway there — it’s not exactly a musical, and it’s only “Hollywood” in the literal sense. But it is a whimsical, lyrical journey through a national heritage suggested and fulfilled by the songs that hold it together.