You can now download clips from the ROTK soundtrack at TORN. I downloaded them all, of course. And, yes: if Howard Shore's work is any indication, they will slay us come December. Beautiful, beautiful stuff.
"Shelob's Lair" is scary and shrieky in a "Psycho"-soundtrack kind of way; "The End of All Things" is a dark and amazing choral arrangement a la Carmina Burana which gave me goosebumps and should impress any music lover, regardless of whether or not they know the story; and, oh, jeez, just try to listen to "The Grey Havens" without getting tears in your eyes. (That last applies mostly just to fangirls like me.)
No, they didn't include the clip where Billy Boyd sings. But I hear he does a good job.