String Quartet in D-Minor op. 56, Voces Intimae.
In the 8th session of David Nice’s excellent survey of the music of Jean Sibelius, he took a time-out from symphonies to explore others of Sibelius’s musical treasures. Here are performances of each:
For more on Luonnotar and the story it tells, see In the Beginning, Cracked Duck Eggs.
Prufrock's Dilemma 
It’s been so long since I heard Voces Intimae that I might as well declare it new to me. And now a favorite. I’m finally growing into the idiom, if that is what it is, of late Romanticism in general and Sibelius’s in particular. Maybe the last frontier, for me! (late 20th and 21st century “isms” have met with less resistance) Must come back to hear your other “treasures” later.
Curt: I am so pleased you enjoyed this–music is always more rich an experience when shared with a good friend! I haven’t anything like your depth or breadth of experience, as you know, with string quartets–yet these days, I can’t seem to get enough of them. So I was just thrilled that David featured this in his current class and found this incredible ensemble performing it. It seems to me to offer quite a different dimension of Sibelius’s compositional art. I am left wondering why he didn’t write more string quartets after this. It would would have been fascinating to see where he went with this form.