Actually, by Jan Garbarek and the Hilliard Ensemble.
First, some background. Jan Garbarek, for those of you unfamiliar with his work, is a Norwegian sax player who plays jazz in sort of the same sense that Keith Jarrett does. Which is to say, his particular kind of improvisational musical expression isn't derived exclusively or primarily from the blues, but also from his own idiosyncratic musical experience (as Jarrett's appears to be derived from his deep appreciation of classical music as much as the "traditonal" jazz masters). He's famously a cold, austere player - he likes to play long notes and his compositions tend to have a lot of aural "space" to them. He's not afraid of silence, or slow, repetitive motifs that sort of surreptitiously build into more complex and interesting things over time. He's a favorite of mine, and he doesn't sound like anyone else I can think of.
This, incidentally, points up something to think about regarding American music. American music - at least, modern American popular music - is just about unthinkable to me without some reference to the blues. The kinds of chord changes and phrases we're conditioned to hear in pop music today are largely derived from the blues. Jazz can be as cerebral and difficult as any music out there, but it's usually still rooted in the blues. We don't, I think, pay this much mind in a typical day, but it struck me forcibly listening to this disk, which features the Hilliard Ensemble singing a collection of medieval chants (exotic and odd sounding to me today I think precisely because they have nothing to do with the blues) while accompanied by Jan Garbarek (who, as I mentioned, has his own unique style of playing that is "jazz" only in some technical or formal sense - it has relatively little to do with (say) Louis Armstrong or Miles Davis).
So, what does Officium sound like? It sounds kind of odd, actually. The whole thing sounds like an unlikely concept-art project, melding chant and saxophone improvisations. Not unlike some kind of weird cousin of a mash-up album in some respects. And in fact, the liner notes make a case for the existence of chant as being a sort of fossilization of an earlier, freer, more improvisational vocal style that died when it was written down. The author of the liner notes explicitly compares chant to The Odyssey, that work being another example of an oral tradition that was frozen when it became written down. It's certainly true, in a slightly different context, that musicians of the Baroque period were in the habit of playing small improvisational passages during composed works, and that that practice died out later on as people came to expect that the musicians would play works exactly as written. Perhaps the same thing happened with chant, too. It's hard to know, and it's pretty much pure speculation on the part of the author of the liner notes. But it is the thread that ties together the apparently random juxtaposition of Garbarek and the Hilliard Ensemble.
As to what it sounds like... again, it's hard to describe. I have yet to get past the weirdness of hearing the saxophone accompanying the chant. In some ways, Garbarek is the perfect guy to play with an early-music group, since he's not the kind of player who steps on the toes of the rest of the band. He leaves a lot of space, and in general his playing emphasizes or accentuates the movement of the voices in the chant. It's an interesting effect, and it was well worth the super-cut-rate used-bargain-bin price I paid for it. Sometimes, though, the sax seems like an intrusion on music that, after all, was never intended to be accompanied in the first place. The album as a whole has the feel to me of a partially-successful experiment. Which, it turns out, was repeated: the same performers teamed up for a disc called Mnemosyne that I'm fairly curious about.
In short, I like the album, but I can't say I'd recommend it to anyone unless they already had an interest in chant, or Jan Garbarek's sax playing, or better yet both. It might also be worth a listen if you like really unusual music. On the other hand, if you're just looking for some nice background music for work, or some rockin' tunes for a road trip - and sometimes, that's just what the doctor ordered - probably best to look elsewhere.