Two initial points came to mind right away. The first is the most simplistic, but is still relevant, and that's that Tolkien took so long to write LotR it was almost inevitable that his 'voice' would change; there is certainly a marked difference in tone between the early chapters of Fellowship and RotK. Of course this argument depends on whether we can detect a return to the original writing style of Fellowship in the final chapters of RotK.
The second thing that springs to mind, and its something that's struck me for a while now, following the CbC read through. There's always a Hobbit present. We don't see Middle-Earth through any eyes but Hobbit eyes. Do we also read Hobbit words? If so, then the tone and style could have been set by the Hobbit observers.
It might be worth looking at which chapters are written in which style. For instance, is all the 'high-flown' language found in the chapters which are viewed through Merry's eyes at Pelennor? Or is it also found elsewhere? And who relates the chapters about Elves? Is it Sam? Are the Elves really like he says?
It could be a whole new area to explore.
I have to disagree that the chapters set in The Shire are 'realism' though. They strike me as more Romantic than the rest of the text to be honest; you could almost imagine Dorothy Wordsworth sitting in the garden of Bag End.
I find the most 'realism' to be in the chapters involving Frodo and Sam's struggles through Mordor.