I finally did what I should have done years ago and exposed myself to this wonderful piece. I picked up the following recording:

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... by the New Helsinki Quartet.

The move away from Sibelius' orchestral music (where I feel at home) and into this more intimate genre was not without aprehension, but I soon found that my worries were unfounded. I have found that although Sibelius is in quite a different "sound world" here, it is still very much Sibelius, and I look forward to learning more about this piece and this style!

Yes, it is puzzling why Sibelius never turned to the quartet medium again, apart from the 'Andante Festivo' best known in its revised version for strings and timpani [JS 34b] but it was originally composed for string quartet in 1922 [JS 34a]
Sibelius's only mature quartet [Voces Intimae] was begun in 1908 and completed during his visit to London in march the following year. So it sits between the 3rd and 4th symphonies and around the time of the two Shakespeare songs and 'In memoriam'.
Ringbom tells us that Sib pencilled in the words 'Voces Intimae' over the three ppp chords of E minor at bar 21 in a copy of the score, at a point that displays a rapt, and withdrawn quality.

I feel that the first movement has distinct resemblances to the 6th symphony, both share the tonal centre of D minor [Dorian] and it has been pointed out that, as a structure it approaches the first movement of the 3rd symphony in perfection of form. In the second movement I always feel that the forest sprites are never far away and it builds up to a powerful climax. In the beautiful third movement 'Adagio di Molto' Sibelius seems to me to move ever closer to the Largo of the 4th symphony. After the Allegretto ma pesante, the Finale drives forward with tremendous momentum to its masterly conclusion.
I cannot agree with those who say that Sibelius thought in orchestral terms when he penned this masterpiece, it is as idiomatic as the quartets of Haydn.--kullervopete.

I've come across an interesting quote about 'Voces Intimae' from a letter that Sibelius wrote to Aino, presumably shortly after its completion.

'It turned out as something wonderful. The kind of thing that brings a smile to your lips at the hour of death, I will say no more'.

What a marvellous thought.--kullervopete.

There is something very familiar right in the beginning, if I remember right without a score.

If I remember rightly, that exceedingly familiar something at the very beginning forms the motivic backbone of the entire piece!

That exceedingly familiar something is also a sort of inverted S-motif...
(see Harri M's excellent other post on this subject)

'It turned out as something wonderful. The kind of thing that brings a smile to your lips at the hour of death, I will say no more'.

What a stange, haunting quote!

Voces intimae is a challenging work and one, I must admit, that I do not know as well as I should. It's strangely dark and mysterious, like much of the master's work.

I'm gonna listen to it tonight...